Add Slovenian geoid model SLO-VRP2016/Koper

[AlexBass05]

This PR adds the Slovenian geoid model SLO-VRP2016/Koper published by the Slovenian geodetic authority GURS (http://www.gu.gov.si).

EPSG v10.081 added the required transformations for supporting this geoid model in PROJ:

• https://epsg.org/transformation_10247/Slovenia-1996-to-SVS2010-height-1.html
• https://epsg.org/transformation_10248/Slovenia-1996-to-Slovenia-1996-SVS2010-height-1.html

The ASCII file used to generate the GeoTIFF was downloaded from here: https://www.e-prostor.gov.si/podrocja/drzavni-koordinatni-sistem/vertikalna-sestavina/?acitem=1257-1258

The license (CC BY 4.0) of the grid file is specified here: https://www.e-prostor.gov.si/en/access-to-geodetic-data/?acitem=1934-1931

Additional information can also be found here: https://isgeoid.polimi.it/Geoid/Europe/Slovenia/slovenia2016_g.html

Corresponding PR in PROJ: https://github.com/OSGeo/PROJ/pull/3610

If this PR finds approval, I'm happy to run regenerate_index_html.py and add an additional commit to this PR.

[rouault]

Excellent. Merging. I'll take care of regenerating the index

[AlexBass05]

Excellent. Merging. I'll take care of regenerating the index

Thanks a lot @rouault