Merge GRIB files

[victoria-cherkas]

If we want to plot / work with multiple grib files, ie timeseries, then it would be useful to have single ds with all data. eg using cfgrib and https://docs.xarray.dev/en/stable/generated/xarray.combine_by_coords.html

[regDaniel]

Thanks for opening this issue, it would be a nice feature to have! @clairemerker also found out that there are performance issues when merging datasets which were loaded with the cfgrib engine (see here). However we didn't yet check whether this is relevant for us and should probably do a small test before starting with the implementation.