refactor .keep_all -> .mustkeep
Add support for assign_to = [rb], keep track of mustkeep in file key/value pairs, better constraint handling and violation checking in assign.py. add a max_priority function to field.py to get the maximum priority of the targets
Make targets link to their setup. This is ok since get_plate does not return a singleton instance of the field.
Enhance graphcollide to support undroppable targets and remove (unused) default weighting based on node order. Use enhancements in assign.py
Speedboost
Get assign_with fully working, adding keepall option to .setup to not drop targets when assigning multiple setups together.