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.

As on import of EPSG, we remove the supersession of Canadian NTv1 file w.r.t NTv2
(because the default behaviour of PROJ is to ignore superseded operations). However
the NTv1 operation is advertized with an accuracy of 1m, whereas NTv2 is advertized
with 1.5m. Consequently on areas where both files are valid, and if both files are
available, NTv1 would be selected. So as a workaround, worsen the NTv1 accuracy to
2m so that NTv2 is used in priority.

Relates to https://github.com/OSGeo/PROJ/issues/1808

Given in.txt with
53.999759140 5.144478208 252.6995

Before the fix,
cct  -t 0 -d 4 +proj=pipeline +step +proj=axisswap +order=2,1,3,4 +step +proj=hgridshift +inv +grids=rdtrans2018.gsb +step +proj=vgridshift +grids=naptrans2018.gtx +step +proj=sterea +lat_0=52.156160556 +lon_0=5.387638889 +k=0.9999079 +x_0=155000 +y_0=463000 +ellps=bessel in.txt

returned:
  139079.8814    668306.0302      212.1724        0.0000

It now returns:
  139079.8850    668306.0458      212.1724        0.0000

which meets with the 1mm accuracy the expected result of test point
```
30010049    53.999759140    5.144478208     252.6995        139079.8850     668306.0460     212.1723
```

 - Tests should use unified diff with expected results
 - Show CMake build compile commands (VERBOSE=1)
 - Travis CI only has two cores, so "-j2" is sufficent

All other tests use PROJ_LIB, and allow it to be overridden from the
command-line, so do the same here.