The Geographic offsets transformation adds an offset to the geographic longitude,
latitude coordinates, and an offset to the ellipsoidal height.
This method is normally only used when low accuracy is tolerated. It is documented
as coordinate operation method code 9619 (for geographic 2D) and 9660 (for
geographic 3D) in the EPSG dataset.

It can also be used to implement the method Geographic2D with Height Offsets
(code 9618) by noting that the input vertical component is a gravity-related
height and the output vertical component is the ellispoid height (dh being
the geoid undulation).

It can also be used to implement the method Vertical offset (code 9616)

It is used for example to transform:
- from the old Greek geographic 2D CRS to the newer GGRS87 CRS
- from Tokyo + JSLD69 height to WGS 84
- from Baltic 1977 height to Black Sea height

It is also useful to document the implicit zero-offset transformation
we do in pipelines such as

+proj=pipeline +step +inv +proj=longlat +ellps=A
               +step +proj=longlat +ellps=B

that can be explicited as

+proj=pipeline +step +inv +proj=longlat +ellps=A
               +step +proj=geogoffset [+dlon=0 +dlat=0 +dh=0]
               +step +proj=longlat +ellps=B

Implement the Equal Earth projection (closes #1085) 

We are relying more and more on C99 math functions. On C89 systems where
those functions are not available our own custom versions of those
functions are used instead. So far these has been spread across the code
base. This commit gathers them in the same file and introduces the
proj_math.h header. The build system checks for C99 math functions. If
not found the proj_math.h header make sure that C99 functions are
defined as their pj_ equivalent.

Ideally proj_math.h is included instead of math.h. This removes the need
for any checks against HAVE_C99_MATH in the code making it easier to
read.

For this commit the functions hypot, log1p and asinh has been taken care
of.

* Call trans func of same dimensionality as input in gie

* Refactor prep/fin code for pj_fwd/pj_inv 2D,3D,4D

* Remove prime meridian handling from pj_transform (now handled in pj_fwd_prepare/pj_inv_finalize)

* Introduce prep/fin skips, mostly in support of axisswap and pipeline drivers

* Refactor fwd/inv subsystem

* pj_transform: Let pj_fwd/inv handle scaling

* Let pj_fwd/inv3d fall back to 2D eventually

* Enable wildcard globbing for MSVC builds

* Use globbing to run all gie tests

* Despite merge title: Also use wildcards on gie tests when using GNU compilers

* Free format now in cmd lines, in gie, and in init files

* Corrected handling of defaults

* Add demo of integrated definition and validation

* Repair stack-smashing memmove in get_init

* repair paralist corruption, clean up debug output

* Install test files for nmake builds

* Add many improvements following suggestions by @schwehr

* Be consistent in requiring lower case everywhere in gie.c

Also, this Fixes #703 and Fixes #697 

Central conic projection implemented as 'ccon'.