Implement top level functions for managing output files

[arbennett]

One candidate:

def merge(file_list: list[str]) -> xr.Dataset:
    out_ds = []
    for f in file_list:
        ds = xr.open_dataset(f)
        ds['hru'] = ds['hruId']
        ds['gru'] = ds['gruId']
        out_ds.append(ds)
    return xr.merge(out_ds)