Prepare 0.26.0 release

[veenstrajelmer]

Todo:

• units attribute is not added during conversion of CMCC #964
• Improve interpolation of uds/ugrid to plipoints #915
• disable waterlevel in cmems_nc_to_ini() #969
• update whatsnew for skipping all quantities except salinity/temperature and tracer* #970
• unable to download ERA5 data using dfmt.download_ERA5 #971
• deprecate dfmt.open_dataset_extra() #973
• abort slow era5 tests #975
• assert two timesteps in dfmt.cmems_nc_to_ini() dataset #977
• fix inconsistent figure scaling with dfmt.plot_coastlines() vs ctx.add_basemap() #966
• nans in modelbuilder salinity bc figure #910
• increase minimal xugrid version to speed up xu.open_dataset() #968

General:

• select more important issues
• check pytest warnings
• merge PR's
• run entire testbank
• update whatsnew

Follow-up: #985