deformation: add support for +grids= for GeoTIFF grids

This option is to load grid(s) that have both the horizontal and
vertical velocities in the same file.

Can be tested with the following grid
https://github.com/rouault/sample_proj_gtiff_grids/blob/master/nkgrf03vel_realigned.tif
converted from the original .ct2 and .gtx with
```
gdal_translate nkgrf03vel_realigned.vrt nkgrf03vel_realigned.tif -co COMPRESS=DEFLATE -co PREDICTOR=3 -co BLOCKYSIZE=241 -co INTERLEAVE=BAND
```

where nkgrf03vel_realigned.vrt is
```
<VRTDataset rasterXSize="223" rasterYSize="241">
  <SRS dataAxisToSRSAxisMapping="2,1">GEOGCRS["Unknown based on GRS80",
    DATUM["Unknown based on GRS80",
        ELLIPSOID["GRS 1980",6378137,298.257222101,
            LENGTHUNIT["metre",1]]],
    PRIMEM["Greenwich",0,
        ANGLEUNIT["degree",0.0174532925199433]],
    CS[ellipsoidal,2],
        AXIS["geodetic latitude (Lat)",north,
            ORDER[1],
            ANGLEUNIT["degree",0.0174532925199433]],
        AXIS["geodetic longitude (Lon)",east,
            ORDER[2],
            ANGLEUNIT["degree",0.0174532925199433]]]
</SRS>
  <GeoTransform>  2.9166666666666670e+00,  1.6666666666666666e-01,  0.0000000000000000e+00,  7.3041666666666686e+01,  0.0000000000000000e+00, -8.3333333333333329e-02</GeoTransform>
  <Metadata>
    <MDI key="DESCRIPTION"></MDI>
    <MDI key="area_of_use">Nordic and Baltic countries</MDI>
    <MDI key="AREA_OR_POINT">Point</MDI>
    <MDI key="TIFFTAG_COPYRIGHT">The Nordic Geodetic Commission. Creative Commons Attribution 4.0 https://creativecommons.org/licenses/by/4.0/</MDI>
    <MDI key="TIFFTAG_DATETIME">2019:12:30 00:00:00</MDI>
    <MDI key="TIFFTAG_IMAGEDESCRIPTION">Deformation model covering the Nordic and Baltic countries. Used in transformations between global reference frames and the local realisations of ETRS89 in the Nordic and Baltic countries</MDI>
    <MDI key="TYPE">VELOCITY</MDI>
  </Metadata>
  <VRTRasterBand dataType="Float32" band="1">
    <Description>east_velocity</Description>
    <UnitType>mm/year</UnitType>
    <SimpleSource>
      <SourceFilename relativeToVRT="0">/home/even/proj/proj-datumgrid/europe/nkgrf03vel_realigned_xy.ct2</SourceFilename>
      <SourceBand>2</SourceBand> <!-- GDAL exposes the first physical component of the file (longitude offset normally, here east velocity) as the second band -->
      <SrcRect xOff="0" yOff="0" xSize="223" ySize="241" />
      <DstRect xOff="0" yOff="0" xSize="223" ySize="241" />
    </SimpleSource>
  </VRTRasterBand>
  <VRTRasterBand dataType="Float32" band="2">
    <Description>north_velocity</Description>
    <UnitType>mm/year</UnitType>
    <SimpleSource>
      <SourceFilename relativeToVRT="0">/home/even/proj/proj-datumgrid/europe/nkgrf03vel_realigned_xy.ct2</SourceFilename>
      <SourceBand>1</SourceBand> <!-- and the second physical component (latitude offset normally, here north velocity) as the first band -->
      <SrcRect xOff="0" yOff="0" xSize="223" ySize="241" />
      <DstRect xOff="0" yOff="0" xSize="223" ySize="241" />
    </SimpleSource>
  </VRTRasterBand>
  <VRTRasterBand dataType="Float32" band="3">
    <Description>up_velocity</Description>
    <UnitType>mm/year</UnitType>
    <SimpleSource>
      <SourceFilename relativeToVRT="0">/home/even/proj/proj-datumgrid/europe/nkgrf03vel_realigned_z.gtx</SourceFilename>
      <SourceBand>1</SourceBand>
      <SrcRect xOff="0" yOff="0" xSize="223" ySize="241" />
      <DstRect xOff="0" yOff="0" xSize="223" ySize="241" />
    </SimpleSource>
  </VRTRasterBand>
</VRTDataset>
```

1 files changed, 32 insertions, 1 deletions

diff --git a/docs/source/operations/transformations/deformation.rst b/docs/source/operations/transformations/deformation.rst
index 3a9d025c..b45c5e2b 100644
--- a/docs/source/operations/transformations/deformation.rst
+++ b/docs/source/operations/transformations/deformation.rst
@@ -35,6 +35,9 @@ GTX format. Both grids are expected to contain grid-values in units of
 mm/year. GDAL both reads and writes both file formats. Using GDAL for
 construction of new grids is recommended.
 
+Starting with PROJ 7.0, use of a GeoTIFF format is recommended to store both
+the horizontal and vertical velocities.
+
 Example
 -------------------------------------------------------------------------------
 
@@ -93,7 +96,7 @@ Parameters
     grid is considered optional and PROJ will the not complain if the grid is
     not available.
 
-    Grids for the horizontla component of a deformation model is expected to be
+    Grids for the horizontal component of a deformation model is expected to be
     in CTable2 format.
 
 .. option:: +z_grids=<list>
@@ -105,6 +108,34 @@ Parameters
     Grids for the vertical component of a deformation model is expected to be
     in either GTX format.
 
+.. option:: +grids=<list>
+
+    .. versionadded:: 7.0.0
+
+    Comma-separated list of grids to load. If a grid is prefixed by an `@` the
+    grid is considered optional and PROJ will the not complain if the grid is
+    not available.
+
+    Grids should be in GeoTIFF format with the first 3 components being
+    respectively the easting, northing and up velocities in mm/year.
+    Setting the Description and Unit Type GDAL band metadata items is strongly
+    recommended, so that gdalinfo reports:
+
+    ::
+
+        Band 1 Block=... Type=Float32, ColorInterp=Gray
+            Description = east_velocity
+            Unit Type: mm/year
+        Band 2 Block=... Type=Float32, ColorInterp=Undefined
+            Description = north_velocity
+            Unit Type: mm/year
+        Band 3 Block=... Type=Float32, ColorInterp=Undefined
+            Description = up_velocity
+            Unit Type: mm/year
+
+    .. note:: :option:`+grids` is mutually exclusive with :option:`+xy_grids`
+              and :option:`+z_grids`
+
 .. option:: +t_epoch=<value>
 
     Central epoch of transformation given in decimalyears. Will be used in