Added `flatten` keyword argument to LogicalTieSelectorCallback. NOTE.

NOTE: You can now choose whether to group selected logical ties by their
originating expression, or to flatten them into a single selection.

EXAMPLE: The following selector selects the first leaf of the first logical tie
in every rest-delimited run of notes and chords:

    selector = selectortools.Selector()

    # select all leaves:
    selector = selector.by_leaves()

    # select runs of notes and chords:
    selector = selector.by_run((Note, Chord))

    # select logical ties, grouped by each run
    selector = selector.by_logical_tie(flatten=False)

    # select the first logical tie of each group
    selector = selector[0]

    # select the first leaf of each logical tie
    selector = selector[0]

More concisely:

    selector=selectortools.Selector(
        ).by_leaves(
        ).by_run((Note, Chord)
        ).by_logical_tie(flatten=False
        )[0][0]