v2.0.0a6

Added

• Keyboard.main_pitch, Keyboard.add_root, Keyboard.key_region #92.
• Sampler.filter and Filter #99.

Changed

• Channel.group becomes a read-only property (modify event to change channel group).
• PLItemBase.offsets and its fields in PlaylistEvent are float32
  Thanks to chrslg from Stackoverflow and @jubabrut.
• Track.height returns an str of its percentage e.g. 100%.
• Instrument.plugin and Slot.plugin return _PluginBase for unimplemented
  native plugins #102.
• Reimplemented EventTree to use a list and got a 10+% perf boost in unit tests.

Fixed

• Channel.group remained unitialised #100.
• Chanel.plugin failed due to base class type parameter check #101.

Removed

• Track.locked_height as what this quantity stores is unknown to me yet.
• Use of fixture factories in unittests #74.