Modify autobeam and grouping properties

Modify autobeam and grouping properties

Create new beamSettings property which stores all information
 about beam creation.

Elminate beatGrouping property; grouping is now defined by the
 default autobeam rule

Add \overrideBeamSettings and \revertBeamSettings to allow the
 complete definition of beamSettings for a time signature in one
 call.

Eliminate the need to revert predefined beam settings in order to
 get the desired autobeaming behavior.

Create new c++ functions (callable from Scheme) to get settings
 out of the beamSettings property.

Update documentation and snippets to reflect the change.