avid mxf: generalize check for avid essence element key
avid mxf: generalize check for avid essence element key

Limecraft/ebu-libmxf,stuarthicks/libmxf,Limecraft/ebu-libmxf,stuarthicks/libmxf,ebu/ebu-libmxf,stuarthicks/libmxf,ebu/ebu-libmxf,stuarthicks/libmxf,Limecraft/ebu-libmxf,ebu/ebu-libmxf,ebu/ebu-libmxf