Improved StaffLineSpanner formatting at end of score.
Improved BuildApiScript.

All parameterized tests pass again.
UpdateManager._update_now() now consults the component parentage.

If any parent of the component to be updated has had their
`_is_forbidden_to_update` flag set to True, then `_update_now()` will bail.
Improved systemtools.Timer documentation.
Added systemtools.ForbidUpdating context manager.

ForbidUpdating is a convenience for preventing offset updates within a logical
block of code, but returning to normal once completed.

    with systemtools.ForbidUpdating():
        mutate(my_score).rewrite_meter((4, 4))
Improved ForbidUpdating context manager.
Merged master.