Implemented callable pitchtools.Inversion class. NEW FUNCTIONALITY.

Object model of the twelve-tone inversion operator.

Example:

    >>> pc = pitchtools.NumberedPitchClass(1)
    >>> inversion = pitchtools.Inversion()
    >>> inversion(pc)
    NumberedPitchClass(11)

Use as persistable way of modeling twelve-tone operators.

Transposition and Multiplication classes to follow.

NEW: should be included in the 2.16 release notes.