rename Tempo _beats_per_minute to _note_types_per_minute, provide pulse helpers.

rename Tempo _beats_per_minute to _note_types_per_minute, provide pulse helpers.

	- adds quarter_notes_per_minute(), note_divisions_per_minute (double)
	  pulses_per_minute() and frames_per_quarter_note()

	- this should be a no-op except for the use of tempo by
	  the vst callback which definitely uses quarter notes per minute.

	- the XML node for TempoSection named 'beats-per-minute'
	  has been renamed.