Delft3D4 related improvements

[veenstrajelmer]

• remove grid variable and attrs in dfmt.open_dataset_delft3d4() #826
• maybe consider using sgrid instead of ugrid: https://github.com/NOAA-ORR-ERD/gridded/tree/master/gridded/pysgrid >> no, ugrid now works quite well
• consider xu.UgridDataset.from_structured although this might not see the cell bounds properly. >> 2d variant will be available
• use xugrid.UgridDataset.from_structured2d in dfm_tools #1125
• include Delft3d4 mdf settings in docstring of function
• enable vertical/horizontal slicing by exposing vertical layer attribute from grid variable (inconvenient name) and make related functions more flexible.
• https://github.com/JulesBlm/pyDelft3D-FLOW
• add attrs and maybe other improvements from https://github.com/Deltares-research/decoimpact_examples/blob/main/preprocessing/Delft3D4_conv_to_Ugrid_Netcdf.ipynb