"unknown" is a valid value for geodeticDatum

[MattBlissett]

From http://rs.tdwg.org/dwc/terms/index.htm#geodeticDatum:

'''The ellipsoid, geodetic datum, or spatial reference system (SRS) upon which the geographic coordinates given in decimalLatitude and decimalLongitude as based. Recommended best practice is use the EPSG code as a controlled vocabulary to provide an SRS, if known. Otherwise use a controlled vocabulary for the name or code of the geodetic datum, if known. Otherwise use a controlled vocabulary for the name or code of the ellipsoid, if known. If none of these is known, use the value "unknown".'''

At present, unknown would get the issue GEODETIC_DATUM_INVALID