Taught RestRhythmMaker about 'spell_metrically' property.

OLD:

    >>> maker = rhythmmakertools.RestRhythmMaker()
    >>> maker([(3, 4), (6, 8)])
    [Selection(Rest('r2.'),), Selection(Rest('r2.'),)]

NEW:

    >>> maker = rhythmmakertools.RestRhythmMaker(
    ...
    Display all 286 possibilities? (y or n)
    ...
    ...     duration_spelling_specifier=rhythmmakertools.DurationSpellingSpecifier(
    ...         spell_metrically=True,
    ...     ),
    ... )
    >>> maker([(3, 4), (6, 8)])
    [Selection(Rest('r4'), Rest('r4'), Rest('r4')), Selection(Rest('r4.'), Rest('r4.'))]

MODEL: this is the beginning of the process of teaching the rhythm-makers how
(optionally) to notate according to common practice conventions of meter.