UNST-9149: Cleanup layertype v2

[thomas-pijls]

What was done

The goal was to keep the (not used) layertypes 3 and 4 in the code, but to clarify their definition and make them private (i.e. remove all reference to these layertypes from public sources such as the User Manual). This was achieved by:

• Layertypes 3 (polygon controlled z-layers) and 4 (density controlled sigma-layers) definitions were clarified
• The layertype 3 and 4 descriptions were updated in all files
• Layertype 3 and 4 specific variables/keywords (VertplizFile, Tsigma, dkx, sdkx) documentation updated
• VertplizFile and Tsigma mdu keywords were set to deprecated and obsolete, respectively
• *.mdu files in repo were updated to remove VertplizFile keyword and updated Layertype description
• User Manual was updated to remove all mention of layertypes 3 and 4 and remove the description of the layertype 3 specific VertplizFile keyword

Evidence of the work done

• Not applicable

Tests

• Not applicable

Documentation

• User manual updated; updated layertype description and removed description of VertplizFile
• Updated keyword description of Layertype and removed description of VertplizFile and Tsigma descriptions in the D-HYDRO-File-Specifications repo

Issue link

[arthurvd]

Thanks, good work across many files. See my comments (and probably make some more changes in the manual appendix too to be consistent with these suggestions)