Make sure "colvars" interoperates stably and smoothly with gromacs - Redmine #3357

The Collective Variables Interface (Colvars) provides a huge number of algorithms that are based on collective variables.

To make sure colvars works smoothly with GROMACS we should work to establish a stable interface that allows to

- access system information over the course of the simulation efficiently

• add to forces, virial, energy
• hook up colvars to gromacs easily

A working version is stored here:

https://github.com/Colvars/colvars/pull/190

(from redmine: issue id 3357, created on 2020-01-28 by cblau)