Temporal gridshifts allow [h|v]gridshift operations to be used as step functions
in a pipeline. This is useful in transformations dealing with deformations caused
by earthquakes.

See the included documentation for details.

Increase the focus on PJ_COORD as primary datatype: Eliminate use of PJ_TRIPLET etc.

Trim proj.h by removing material that has become unnecessary.

Improve constness

Make proj_factors work in proj.h space, by providing a trimmed down PJ_FACTORS (which even has more meaningful field names)

* Removed remaining traces of the builtin selftest system. Moved all functionality to test/gie

* Updated Appveyor and Travis build scripts

* Another appveyor script update

* Poder dual autochecking implementation
* Debugging aid: Improvements in PJ_vgridshift.c and gie.c

* Most likely, the bugbeing tripped is in the gridshift code, so. uncomment suspicious lines in deformation.gie and merge this to support the debugging effort

PJ_OBS eliminated, API adjusted to reflect that we now have only one 4D data type.
2 new API functions added to determine output types of a PJ.

This refactoring of the grid shift functions allows for easier access to
the actual grid values, as well as making it possible to perform
horizontal grid shift on a single coordinate without using the
pj_apply_gridshift* functions. The latter simplifies the execution path
of the forward and inverse functions in PJ_hgridshift.c.

This commit introduces proj_*grid_init, proj_*grid_value
and proj_hgrid_apply. The init functions initialises horizontal and
vertical grids respectivelive (wrappers for pj_gridlist_from_nadgrids
with simpler parameters). The proj_*grid_value functions returns the
specific grid value at coordinate lp. The proj_hgrid_apply function
applies the grid offset to the input coordinate and outputs the adjusted
coordinate.