Added rhythmmakertools.mask_all() function.

The function is an alias to make a boolean pattern that specifies that all
elements should be masked.

Useful when making, for example, a rest rhythm-maker:

    >>> mask = rhythmmakertools.mask_all()
    >>> maker = rhythmmakertools.NoteRhythmMaker(
    ...     output_masks=[mask],
    ...     )

NEW: should be noted in the 2.16 release notes.