Recommendation: Create Custom Force Field(Potential) inside the folder "functionTerms"

we want to keep the main backbone file "openmmawsem.py" stable.(such that everyone has the same "openmmawsem.py) It's better you develop the new force field in your local folder(where you run your program, not the github folder), after you finalized the force field, move it to the "functionTerms" folder under your github folder and commit to the github server.

Please make sure your openMM version is at least 7.3.1

It fixed two bug we found.