Extended DurationSpellingSpecifier spell_metrically with 'unassignable' option.

You can now use ...

    duration_spelling_specifier=rhythmmakertools.DurationSpellingSpecifier(
        spell_metrically='unassignable',
    )

... to spell unassignable durations like 5/8 and 9/8 metrically (ie, broken
down into parts) while leaving assignable durations like 3/8 and 4/8 in tact.

NEW: note that this is a a very good approximation of common era practice for
filling measures with rests.