Harmonized HypermeasureDivisionMaker property names.

OLD:

    HypermeasureDivisionMaker.division_maker
    HypermeasureDivisionMaker.hypermeasure_specifier

NEW:

    HypermeasureDivisionMaker.hypermeasure_callback
    HypermeasureDivisionMaker.hypermeasure_specifier

The new name 'hypermasure_callback' clarifies the fact that what the maker is
does is to first glue input together according to the hypermeasure specifier
and then optionally postprocess the resulting hypermeasures according to the
hypermeasure callback (which will be yet another type of division-maker).