Implementing the NKG transformations in proj.db

[kbevers]

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.

Note that the nkg.gie file is included in the repo but not added to the test suite (yet?) as it relies on a couple of grids that I don't want to include here and are not smart enough to reduce to a subset that will be suitable for the test-suite.

I intend to back-port this to the 7.2 branch as well, so that the transformations will be available on system where PROJ 7 is an option (I expect some packaging systems will be late in adopting 8.0.0).

[PROJ-BOT]

The backport to 7.2 failed:

The process 'git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-7.2 7.2
# Navigate to the new working tree
cd .worktrees/backport-7.2
# Create a new branch
git switch --create backport-2494-to-7.2
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 1cafe3e602d3f697c8d2daaa9b634f3ad23b0d53
# Push it to GitHub
git push --set-upstream origin backport-2494-to-7.2
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-7.2

Then, create a pull request where the base branch is 7.2 and the compare/head branch is backport-2494-to-7.2.