* Make tmerc an alias for etmerc

This switches the algorithm used in tmerc to the Poder/Engsager
tmerc algorithm. The original tmerc algorithm of Evenden/Snyder
origin can still be accessed by adding the +approx flag when
initializing a tmerc projection. The +approx flag can also
be used when initializing UTM projections, in which case the
Evenden/Snyder algorithm is used as well.

If a tmerc projection is instantiated on a spherical earth
the Evenden/Snyder algorithm is used as well since the
Poder/Engsager algorithm is only defined on the ellipsoid.

+proj=etmerc can still be instantiated for backwards compatibility
reasons.

Co-authored-by: Kristian Evers <kristianevers@gmail.com>
Co-authored-by: Even Rouault <even.rouault@spatialys.com>

c6ab83f5742bc5ac6f9cb9a8b2a4f1ea241b6f63 already removed their availability
in user facing application, but the library code remained, and appeared to be
unused by the rest of the library, and not available to library users,
the API being only in proj_internal.h. So remove all remains.

Remove all traces of nad2bin and nad2nad

The source material for the default grids used by
PROJ has been moved to the proj-datumgrid repository. For that
reason it is no longer necessary to include the nad2bin program
in the PROJ repository and source distribution. From now on the
nad2bin application will be kept in the proj-datumgrid repo.

Previously the null grid was generated by running nad2bin on the
null.lla file. Since nad2bin is no longer available null.lla has
been replaced by its binary counterpart null. This file will be
distributed and installed alongside PROJ.

Build scripts and documenation has been adjusted so that nad2bin
is not mentioned anywhere. Additionally all references to nad2nad
has been removed as well. nad2nad has not been part of the PROJ
distribution for quite some time so this has been long overdue.