fixed handling of restr_fname in refinement modules

[mgiulini]

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Closes #589 by setting the attribute restr_fname to None when models to be refined have yet to be merged

[joaomcteixeira]

This one is deep; I can't consider all the implications of this change by heart without being 100% into the project. I suggest inspecting tests and results with the compare_runs.py script in the examples/. Ping me if any questions about those scripts. 👍

[amjjbonvin]

It should not affect the other tests as this is basically a new scenario in which different restraints sets can be used within the same run. Was required to mimic what the 2.4 server does in the bioinformatics mode, generating custom restraint files for each model in it0. And this new implementation gives much more freedom.