ERA5: fixes and small refactor for loading copernicus era5 netcdf data

Made ERA5 data downloaded from the copernicus archive in netCDF format loadable. This data has the pressure unit in millibars, which has been added. Also the positive attribute has been changed to optional for the pressure dimension (CF conform). CF conventions do not specify all possible pressure units, so we have to add ones we encounter in the future.

Refactored the check for Pa and hPa type units.

Also, I had to remove the consistency check regarding add_offset and scale_factor for netCDF data, since before all variables were required to have the same offset and scale factor. This doesn't make sense or?? Maybe comment on this (see lines 2290ff in ClimateForecastReader). Maybe this was an unintended bug as a result of the previous GRIB reader merge.

This should address #274 (closed) and #292 (closed).