Use binary search in Axis_group_interface::combine_pure_heights.
Originally reported by: hanwen
This used linear search, probably making the cost quadratic in the
score size
In the Carver MSDM score, this function accounts for ~1% of processing
time.
https://codereview.appspot.com/573730043
Original URL: https://sourceforge.net/p/testlilyissues/issues/5926