update dfm_tools after ddlpy update to new waterwebservices

[veenstrajelmer]

Todo:

• waterwebservices will be updated to WADAR and that also slightly changes the DDL API
• update to new RWS WaterWebservices ddlpy#113
• release ddlpy
• require new minimal ddlpy version
• this slightly changes how dfm_tools should communicate with ddlpy, update the code/examples
• also update the dfm_tools code to account for changes in explicitly preserve instead of drop in dataframe_to_xarray() ddlpy#157 and Also drop non-unique .Omschrijving columns in dataframe_to_xarray() ddlpy#159 and explicitly pass df and always_preserve arguments