feat: extract constants and climatologies from a dataset for use in inference

[b8raoult]

Inference will need the constant fields as well as the climatologies (12 fields, one per calendar month) used during training. The new command extract extract these and write them into a NetCDF file, that will be later used by anemoi-inference (to be implemented).

Example:

anemoi-datasets extract --constant z --constant lsm --climatology lai_hv --climatology  lai_lv aifs-rd-an-oper-i6aj-mars-o96-2010-2022-1h-v1-ecland-era5met out.nc

Will extract the constants z (orography) and ism (land-sea mask), and the climatologies lai_lv (leaf area index, low vegetation) and lai_hv (leaf area index, high vegetation) into out.nc

The NetCDF follows the CF convention for climatologies:

netcdf out {
dimensions:
        values = 40320 ;
        time = 12 ;
        nv = 2 ;
variables:
        double latitude(values) ;
                latitude:units = "degrees_north" ;
        double longitude(values) ;
                longitude:units = "degrees_east" ;
        float z(values) ;
        float lsm(values) ;
        double time(time) ;
                time:units = "days since 2000-01-01" ;
                time:climatology = "climatology_bounds" ;
                time:calendar = "gregorian" ;
        double climatology_bounds(time, nv) ;
        float lai_hv(time, values) ;
                lai_hv:cell_methods = "time: mean within years time: mean over years" ;
        float lai_lv(time, values) ;
                lai_lv:cell_methods = "time: mean within years time: mean over years" ;

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