add groupby to `FileContainer`?

[mathause]

We have the following snippet

for scen in scenarios_incl_hist:
    files = fc_all.search(scenario=scen)

    # load all members for a scenario
    for fN, meta in files.items():
        ds = xr.open_dataset(fN)

with groupby we could do the following instead:

for scen, fc_scen in fc.groupby("scen"):

    # load all members for a scenario
    for fN, meta in fc_scen.items():
        ds = xr.open_dataset(fN)

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A "working" example using groupby of the underlying pandas DataFrame:

import filefisher

ff = filefisher.FileFinder("", "{scen}.nc", test_paths=["hist.nc", "ssp126", "ssp585.nc"])
fc = ff.find_files()

for scen, df  in fc.df.groupby("scen"):
    
    for fN, meta in df.iterrows():
        print(fN)