Deprecate FuSGC, add MCBias, use Traces for general state saving in MC

[lbluque]

Summary

• Deprecated FuSemiGrandEnsemble (now implemented as a bias term). Renamed MuSemiGrandEnsemble to SemiGrandEnsemble.
• Implemented MCBias interface (from @qchempku2017), and FugacityBias.
• Renamed MCUshers without the 'er' ending (i.e. Flipper to Flip) to simplify factory construction from string names.
• Implemented Trace and StepTrace to save (occupancies and general state information for MC steps), only ndarrays can be saved though.

Additional dependencies introduced (if any)

• Python 3.7 does not work now, should drop support.

Checklist

• Code is in the standard Python style.
  Run pycodestyle and flake8
  on your local machine.
• Docstrings have been added in the Google docstring format.
  Run pydocstyle on your code.
• Tests have been added for any new functionality or bug fixes.
• All existing tests pass.