Function set_stem_direction_for_beam_groups 

pub fn set_stem_direction_for_beam_groups(stream: &mut Stream)

Points the stems of each beamed group the way its first and last pitches lie about the middle line of the clef in force: music21’s setStemDirectionForBeamGroups.

A group runs from a note whose first beam starts to the one whose first beam stops. Stems a score already points all one way are left alone, and a group with no clef before it is not touched.