removed unused self reference
mostly just a bit of tidying

- added parameter documentation for onMidi function
- added parameter documentation for send_midi
- removed passing the trackbank object to the default callback function in BCF.build_control_layout
fixed the bcr's onmidi call
added the common functionality to bcr2000_object from bcf2000_object

- added several object properties to the constructor
- added documentation to the init function
- added body of init function
- added documentation to the flush function
- added the function body to the onmidi function
- added proper default arguments
- added the build_indexed_controls function
- added the send_midi function
- copied, modified and commented-out the BCF.bind_observers function
- updated the function body and logic of the BCR.build_control_layout function