Defined measure storage format.

Example:

    >>> measure = Measure((5, 12), "c'4 c'8 c'8 c'8", implicit_scaling=True)
    >>> print format(measure, 'storage')
    scoretools.Measure(
        indicatortools.TimeSignature((5, 12)),
        "c'4 c'8 c'8 c'8",
        implicit_scaling=True,
        )

This makes it possible to persist (simple) measures as rhythmic expressions to
disk.

Overrides and spanners do not appear in the storage format. But all rhythmic
information does appear in the storage format.