Added the Mf.test_bug_20213_asn_sidechain() system test to catch bug #20213.
The data and script comes from the files 'sh3-47.2.zip' and 'run.py' attached to the bug report
https://gna.org/bugs/?20213. The PDB now only contains Asp47, the optimisation parameters have been
made almost insignificant, and all models but 'tm0' have been removed from the analysis.