Fixes #1984

- Copy BETA2007.gsb, MD, alaska, conus, ntf_r93.gsb, ntv1_can.dat grids
  from proj-datumgrid to data/tests.
- Replace a couple uses of nzgd2kgrid0005.gsb in tests by ntf_r93.gsb
- Add downsampled/subsetted versions of egm96_15.gtx as tests/egm96_15_downsampled.gtx
  and ntv2_0.gsb as tests/ntv2_0_downsampled.gsb
  This results in a few changes in expected results
- Simpify travis/install.sh due to less configurations to test

This results in a hopefully acceptable increase of the proj-X.Y.Z.tar.gz
from 2.9 to 5.3 MB

And add testing of both little-endian and big-endian NTv2 files

Fix test issues on i386

Fix a few issues of #1906 found when running the test suite
on Ubuntu 16.04 with gcc 5.5 -m32. When applied on top of
the fix of #1912, make check succeeds

This should avoid issues with numerical stability as uncovered in
https://github.com/OSGeo/PROJ/issues/1906.

Practically speaking this change isn't going to affect real life
scenarios since the position of the center of the Earth is rarely
expressed in geodetic coordinates.

Fixes #1846

This commit is the result of the squashing of rfc4_dev branch in a single
commit. It implements mostly RFC 4 related work.

* Grid handling:
  - remove obsolete and presumably unfinished implementation of grid catalog functionality
  - all grid functionality is in grids.cpp/.hpp
  - vertical and horizontal grid shift: rework to no longer load whole grid into memory
  - remove hgrids and vgrids member from PJ structure, and store them in hgridshift/vgridshift/deformation structures
  - build systems: add optional libtiff dependency. Must be explicitly disabled if not desired
  - add support for horizontal and vertical grids in GeoTIFF, if libtiff is available
  - add GenericShiftGridSet and GenericShiftGrid classes, relying on TIFF grids, that can be used for generic purpose grid-based adjustment
  - add a +proj=xyzgridshift method to perform geocentric translation by grid. Used for French NTF to RGF93 transformation using gr3df97a.tif grid
  - deformation: add support for +grids= for GeoTIFF grids
  - horizontal grid shift: fix failures on points slightly outside a subgrid (fixes #209)

* File management:
  - add a filemanager.cpp/.hpp to deal with file related work
  - test for legacy proj_api.h fileapi
  - proj.h: add proj_context_set_fileapi() and proj_context_set_sqlite3_vfs_name() (fixes #866)
  - add capability to read resource files from the user writable directory

* Network access:
  - build systems: add optional curl dependency
  - add a curl-based default implementation for network related functionality
  - proj.h: add C API to control network functionality, and optionaly provide network callbacks
  - add data/proj.ini with default settings
  - add a SQLite3 local cache of downloaded chunks
  - add proj_is_download_needed() and proj_download_file()

* Use Win32 Unicode APIs and expect all strings to be UTF-8 (fixes #1765)
  For backward compatibility, if PROJ_LIB content is found to be not UTF-8 or
  pointing to a non existing directory, then an attempt at interpretating it
  in the ANSI page encoding is done.
  proj_context_set_search_paths() now assumes strings to be in UTF-8, and
  functions returning paths will also return values in UTF-8.

Should fix the issue reported in https://lists.osgeo.org/pipermail/proj/2020-January/009188.html
Some extra north-american grids present in data/ can affect the results of
some tests, so create a data/for_tests/ subdirectory in which we copy only
select grids.

Inspired from syntax of https://github.com/OSGeo/PROJ/pull/453/files
but 'rebased' on top of previous commit that cleans up the pipeline implementation

Different situations:
- +omit_fwd:
    the step when followed in the forward path will be omitted
    the step when followed in the reverse path will be executed
- +omit_fwd +inv:
    the step when followed in the forward path will be omitted
    the step when followed in the reverse path will be executed (with the inv method)
- +omit_inv:
    the step when followed in the forward path will be executed
    the step when followed in the reverse path will be omitted
- +omit_inv +inv:
    the step when followed in the forward path will be executed (with the inv method)
    the step when followed in the reverse path will be omitted

This will be used in the next commit to optimize constructs like

+step +proj=hgridshift +grids=foo
+step +proj=vgridshift +grids=bar
+step +inv +proj=hgridshift +grids=foo

Such steps are used for CRS to CRS transformations where applying the vertical grid
requires to do a transformation to an interpolating CRS. One can notice that
in the last step will just restore the horizontal coordinates before the first step, so
doing an inverse hgridshift is overkill.

So that could be optimized as:

+step +proj=push +v_1 +v_2
+step +proj=hgridshift +grids=foo +omit_inv
+step +proj=vgridshift +grids=bar
+step +inv +proj=hgridshift +grids=foo +omit_fwd
+step +proj=pop +v_1 +v_2

In the forward path, this will be equivalent to:
+step +proj=push +v_1 +v_2
+step +proj=hgridshift +grids=foo
+step +proj=vgridshift +grids=bar
+step +prop=pop +v_1 +v_2

And similarly in the reverse path, this will be quivalent to:
+step +proj=push +v_1 +v_2
+step +proj=hgridshift +grids=foo
+step +inv +proj=vgridshift +grids=bar
+step +proj=pop +v_1 +v_2