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

Fixes #1750

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.

Fixes side-effect of https://github.com/OSGeo/PROJ/issues/1525 that went in 6.1.1

The correction is horribly hacky. Sorry...