Part name get's lost when writing. #13
Description
Hello! Thank you for fantastic work!
Issue
Music21's object part names (stored as part.partName) get lost converting to MEI.
Minimal code example
import music21 as m21
import converter21;
converter21.register()
def get_part_names(score): return [p.partName for p in score.parts]
score = m21.corpus.parse('palestrina')
part_names = get_part_names(score)
print(part_names)
# ['Soprano', 'Alto', 'Tenor', 'Tenor', 'Bass']
score_mei = m21.converter.parse(score.write('mei'))
part_names_none = get_part_names(score_mei)
print(part_names_none)
# [None, None, None, None, None]