Added two methods to PitchClassSegment.

    PitchClassSegment.voice_horizontally()
    PitchClassSegment.voice_vertically()

`voice_horizontally()` converts pitch-classes to pitches in a manner similar to
LilyPond's \relative note entry mode.

`voice_vertically()` converts pitch-classes to pitches with each resulting
pitch higher than the previous.
Merged master.