Auto-beaming changes

Auto-beaming changes

 - add beam grouping rules for 3/8, 4/8, 6/4, 6/16,
   9/4, 9/16, 12/4, 12/16 time signatures

 - amend beam ending rules to limit lengths of
   beamed runs of 16th and 32nd notes in some
   time signatures only (these may need some
   fine-tuning with experience and feedback)

 - except for 4/4 time, where the previous beaming
   is preserved

 - amend documentation to match