This adds the NKG 2008 and 2020 transformations to proj.db. The NKG
transformations offers transformations between global reference frames
and the national realisations of ETRS89 in Denmark, Estonia, Finland,
Latvia, Lithuania, Norway and Sweden.

The 2008 transformations are already implemented in the NKG 2008 file
but will now be more accessible with the modern API.

The 2020 transformations are new to PROJ and offers and updated version
of the 2008 transformations using a new and improved deformation model
(eu_nkg_nkgrf17vel.tif). A 2020 version of the NKG transformations are
currently not available for Norway but will in all likelyhood be
included at a later point in time.

Fixes #2482

And also add proj_context_errno_string()

Revise gie 'expect failure errno XXXX' strings

Add +proj=topocentric geocentric->topocentric conversion (fixes #500)

The restriction was a copy&paste from the Evenden/Snyder approximate
ellipsoidal implementation, but the spherical one is exact, so this
restriction isn't needed.

Also tune a bit the handling of lat=0, |lon| > 90

Corrected formula given by @evanmiller

Due to the improved accuracy of pj_tsfn(), it no longer returns
0 when phi=90° due to the conversion in radians.
Some GDAL tests are very sensitive to the pole transforming to (0,0)
exactly, so add a special case for that.

master only

Update Mercator projection, more accurate, faster