Added detach() method to InspectionAgent.

Example score:

    >>> staff = Staff("c'8 d'8 e'8 f'8")
    >>> clef = marktools.ClefMark('alto')
    >>> attach(clef, staff)

OLD DETACH:

    >>> clef.detach()

NEW DETACH:

    >>> inspect(staff).detach(marktools.ClefMark)

Old usage is now deprecated.