Fix support for electrode location #321

1  BEST_PRACTICE_VIOLATION
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1.1  nwb_files/createElectricalSeries.nwb and 1 other file: check_electrodes_location_allen_ccf - 'NWBFile' object at location '/'
       Message: Electrode location 'unknown' is not a term in the Allen Mouse Brain CCF ontology. Please use either the full name or abbreviation from the Allen Mouse Brain Atlas (e.g., 'Primary visual area' or 'VISp'). This check can be ignored if Allen CCF terms do not meet your needs.