reduce netcdf filesizes

[veenstrajelmer]

optimize netcdf options: zipping, chunking, float32, etc.

Two feasible options identified, convert strings from <Uxx to |Sxx or so (in ddlpy.dataframe_to_xarray()), or write as format="NETCDF4_CLASSIC" in kw (which also converts strings automatically). Since the latter is far simpler, the ddlpy docs were updated in Deltares/ddlpy@abb0d02, we will also just add it to kw.retrieve_measurements()