second 2.0.0 beta release

Bug fixes and improved error checking.

BRER workflow scripts.

https://github.com/kassonlab/run_brer
is now the home of software 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

Updates with this release include

• support for the updated https://github.com/kassonlab/brer_plugin package
• support for GROMACS 2021 and gmxapi 0.2
• various small bug fixes and usability improvements