Implementing the NKG transformations in proj.db

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.

data/Makefile.am

@@ -42,6 +42,7 @@ SQL_ORDERED_LIST = sql/begin.sql \
 		sql/grid_alternatives.sql \
 		sql/grid_alternatives_generated_noaa.sql \
 		sql/customizations.sql \
+		sql/nkg.sql \
 		sql/commit.sql
 
 EXTRA_DIST = proj.ini GL27 nad.lst nad27 nad83 \