Added NumberedPitch.interpolate(stop_pitch, fraction). NEW FUNCTIONALITY.

Example: interpolates up from C4 to C5:

    >>> start_pitch = pitchtools.NumberedPitch(0)
    >>>> start_pitch.interpolate(stop_pitch, Fraction(0))

    NumberedPitch(0)
    >>> start_pitch.interpolate(stop_pitch, Fraction(1, 4))
    NumberedPitch(3)
    >>> start_pitch.interpolate(stop_pitch, Fraction(1, 2))
    NumberedPitch(6)
    >>> start_pitch.interpolate(stop_pitch, Fraction(3, 4))
    NumberedPitch(9)
    >>> start_pitch.interpolate(stop_pitch, Fraction(1))
    NumberedPitch(12)
    >> stop_pitch = pitchtools.NumberedPitch(12)

Example: interpolates down from C5 to C4:

    >>> start_pitch = pitchtools.NumberedPitch(12)
    >>>> start_pitch.interpolate(stop_pitch, Fraction(0))

    NumberedPitch(12)
    >>> start_pitch.interpolate(stop_pitch, Fraction(1, 4))
    NumberedPitch(9)
    >>> start_pitch.interpolate(stop_pitch, Fraction(1, 2))
    NumberedPitch(6)
    >>> start_pitch.interpolate(stop_pitch, Fraction(3, 4))
    NumberedPitch(3)
    >>> start_pitch.interpolate(stop_pitch, Fraction(1))
    NumberedPitch(0)
    >> stop_pitch = pitchtools.NumberedPitch(0)

NEW: should be included in the 2.16 release notes.