Introduced DurationSpellingSpecifier to rhythmmakertools.

The class is currently a stub.

The class will bundle the decrease_durations_monotonically=True and the
forbidden_written_duration=None rhythm-maker input parameters.

Those parameters are now deprecated.

Importantly the class will also provide an object-modeled namespace for future
duration-spelling parameters. We will almost certainly want to incorporate
options for invoking rewrite_meter() on all rhythm-makers at some point. The
parameters specifying how that will happen should be implemented as properites
on DurationSpellingSpecifier.