Accumulation fail for set of fields having different values of "timespan"

[flyIchtus]

When computing accumulation of grib-files, the files having different values for the "timespan" metadata make the accumulation impossible to run.

Example of output:

ValueError: Accumulator not complete: Accumulator(valid_date=2025-01-01 06:00:00, 6h, key={ levtype=sfc, param=tp, timespan=1h })
    done: SignedInterval(20250101.0100+1h ->         0200 )
    done: SignedInterval(20250101.0200+1h ->         0300 )
    done: SignedInterval(20250101.0500+1h ->         0600 )
    done: SignedInterval(20250101.0300+1h ->         0400 )
    done: SignedInterval(20250101.0400+1h ->         0500 )
    todo: SignedInterval(20250101.0000+1h ->         0100 )

The accumulator is not complete, because it has 'timespan=1h' in its keys, but some fields needed for accumualtion have 'timespan=fs'.

A solution would be to remove 'timespan' as a key describing accumulator (similarly to e.g, date or step). Proposed fix in #516