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>
```

 - Use names as published
 - Change Tobler2017 -> Tobler2018
 - Change EPSGGuidanceNumber7Part2 -> IOGP2018

The +t_obs parameter was confusing for users since it effectively
overwrote the observation time in input coordinates. To make it more clear
what is the operation is doing, users are now required to directly specify
the time span for which they wish to apply a given deformation. The parameter
+dt has been added for that purpose. The new parameter is mutually
exclusive with +t_epoch. +dt is used when deformation
for a set amount of time is needed and +t_epoch is used (in
conjunction with the observation time of the input coordinate) when
deformation from a specific epoch to the observation time is needed.

Changed the direction of dt-calculation to follow the same convention as
helmert. Changed from dt = t_c - t_obs to dt = t_obs - t_c, which
effectively reverses the direction of the operation. Existing projstrings
using deformation can simply reverse the direction of the operation to
achieve the same results as before this commit.