Add example showing how to add restraints

[jchodera]

This suggestion is from twitter:

[apayne97]

this exists, no? https://github.com/openmm/openmm-cookbook/blob/main/notebooks/cookbook/Restraining%20Atom%20Positions.ipynb

[peastman]

Maybe the idea is to apply restraints based on a collective variable? @jchodera do you have any insight on what the poster had in mind?

[jchodera]

@peastman : That's my guess: Definition restraints on collective variables.

The Plumed package has much better documentation and sophisticated ways of doing this, though. I wonder if a more complete tutorial on using Plumed with OpenMM might be useful?