Do not implicitly set generatingProcessIdentifier when writing GRIB data

[sandorkertesz]

Fixes #274

Previously, when writing a NumpyFieldList the generatingProcessIdentifier ecCodes GRIB key was always set to 255 (internally) in the output.

With this PR this key is not set implicitly but must be set explicitly in Metadata.override(). E.g.:

# ds is a fieldlist
md = ds[0].metadata().override(shortName="msl", generatingProcessIdentifier=255)
r = FieldList.from_numpy(ds[0].values, md)
r.save("my_data.grib")