Implemented Component._logical_measure_number. NEW FUNCTIONALITY.

You can now call by hand ...

    >>> component._update_logical_measure_numbers()

... and all components everywhere in score will have their new
_logical_measure_number property set correctly to a positive integer.

NOTE: the functionality is currently private.

CAVEAT: the update functionality doesn't work if you have multiple time
signatures in effect at the same time (in different staves, say); the
assumption is that all time signatures basicallys work at the score context.
(We can generalize this later after baking in the existing version of the
functionality.)

Example:

    staff = Staff("c'4 d' e' f' g' a' b' c''")
    attach(TimeSignature((2, 4)), staff[0])
    attach(TimeSignature((3, 4)), staff[4])
    attach(TimeSignature((1, 4)), staff[7])

    staff._update_logical_measure_numbers()
    for note in staff:
        print(note, note._logical_measure_number)
    ...
    (Note("c'4"), 1)
    (Note("d'4"), 1)
    (Note("e'4"), 2)
    (Note("f'4"), 2)
    (Note("g'4"), 3)
    (Note("a'4"), 3)
    (Note("b'4"), 3)
    (Note("c''4"), 4)