Support for 3D tekal files (polyfile format)

[veenstrajelmer]

Is your feature request related to a problem? Please describe.
Some of the tekal-files in the script https://github.com/Deltares/dfm_tools/blob/main/tests/examples/preprocess_hydrolib_readwritepol.py cannot be read since they contain a 3D datablock and that is not supported by hydrolib.

Describe the solution you'd like
It might be nice to support this, but I can imagine it is out of scope.

Describe alternatives you've considered
What also sort of works, is changing the header (so it contains only numcols and numrows) and reshaping the data before plotting, also done in the example script.

[veenstrajelmer]

Discussed with @arthurvd that this will not be implemented.