Issue raised in https://github.com/OSGeo/proj.4/issues/1191.

With projects.h not being available to outside users anymore we need to
define PJ_UNITS, PJ_ELLPS, PJ_PRIME_MERIDIANS and PJ_OPERATIONS
elsewhere.

Related pj_get_*_ref() functions have been removed in favour
of their proj_ namespaced counterparts.

char pointers have been changed to const char pointers.

Resolves #983
Resolved #1147

Make char pointers const

* Avoid using angle brackets for PROJ headers.

Switching from #include <proj.h> to #include "proj.h",
and correspondingly for projects.h and proj_api.h,
reduces the risk of accidentally picking up a system
installed older version of the header while building
a new version of PROJ.

* Also handle geodesic.h

Having non-const variables of static lifetime or even global scope is
usually a bad idea. These variables are inherently constants, and this
should be enforced.

This required marking some functions as not modifying input parameters
and marking some pointers as pointers to const.

One advantage is that the compiler usually puts const static variables
in a read-only code segment, so they can't be modified physically.
This can be verified with `nm` (on POSIX systems).

To avoid changes to the public API, functions returning non-const
pointers to data tables were left intact, but the returned data may not
be modified. Internally we prefer using the proj_list_* functions over
the pj_get_*_ref functions, because the former return const pointers.

Improve error messaging for cct and gie, and do some clean ups in the ellipsoid handling - partially to squash bugs, partially to improve naming consistency which, in turn, improves the readability of the ellipsoid handling code.

Renamed functions:

pj_inherit_ellipsoid_defs has been renamed pj_inherit_ellipsoid_def, while pj_calc_ellps_params has been renamed pj_calc_ellipsoid_params.

The code in get_opt (part of pj_init.c), which handles whether or not an ellipsoid definition should be dragged in from proj_def.dat, has been rewritten. I suspect this was buggy beforehand, and at least the new code is easier to follow (although it may be slightly slower, which is not really a problem as it sits in the setup code, and hence is executed only once).