BRER MD extension code for GROMACS

This release establishes https://github.com/kassonlab/brer_plugin as the home for the BRER MD extension code described in
Hays, J. M., Cafiso, D. S., & Kasson, P. M. Hybrid Refinement of Heterogeneous Conformational Ensembles using Spectroscopic Data. The Journal of Physical Chemistry Letters. DOI: 10.1021/acs.jpclett.9b01407

Significant changes from the initial release:

• The Python module name is now brer (instead of myplugin)
• The project is no longer managed as a fork of https://github.com/kassonlab/sample_restraint
• The package has been updated for compatibility with GROMACS 2021 and gmxapi 0.2.