Activated Measure.should_scale_contents property.

EXISTING DEFAULT BEHAVIOR:

    >>> measure_1 = Measure((5, 12), [])
    >>> measure_1.should_scale_contents = True
    >>> measure_1.extend("c'8 d'8 e'8 f'8 g'8")

    >>> f(measure_1)
    {
            \time 5/12
            \scaleDurations #'(2 . 3) {
                    c'8
                    d'8
                    e'8
                    f'8
                    g'8
            }
    }

NEW FUNCTIONALITY:

    >>> measure_2 = Measure((5, 12), [])
    >>> measure_2.should_scale_contents = False
    >>> tuplet = Tuplet((2, 3), "c'8 d'8 e'8 f'8 g'8")
    >>> measure_2.append(tuplet)

    >>> f(measure_2)
    {
            \time 5/12
            \times 2/3 {
                    c'8
                    d'8
                    e'8
                    f'8
                    g'8
            }
    }

Note that to use the new behavior you set should_scale_contents to false and
then build a (possibly incomplete) tuplet by hand.