Implemented bounded Instrument transposition methods. NEW FUNCTIONALITY.

NEW:

    * Instrument.transpose_from_sounding_pitch_to_written_pitch()
    * Instrument.transpose_from_written_pitch_to_sounding_pitch()

Example:

    >>> note = Note("c'4")
    >>> instrument = instrumenttools.ClarinetInBFlat()
    >>> instrument.transpose_from_written_pitch_to_sounding_pitch(note)
    >>> note
    Note('bf4')

These methods duplicate the functions ...

    * pitchtools.transpose_from_sounding_pitch_to_written_pitch()
    * pitchtools.transpose_from_written_pitch_to_sounding_pitch()

... already present in the API.

The bound methods will probably wind up being the preferred way to access the
functionality.

NEW: should be included in the 2.16 release notes.