Improvements to genutils.decorators subpackage

[timbernat]

• decorators.classmod: add class decorator which makes certain attributes immutable (based on some of the approaches presented here)
• Reimplement register_subclasses decorator using init_subclass
  • Re-implement occurrences of abstract classproperties using the new "register_abstract_class_attrs()" decorator, namely for:
    • jsonio.serialize.TypeSerializer
    • rdconvert.RDConverter
    • openmmtools.thermo.EnsembleFactory
    • molchargers.MolCharger
• decorators.functional
  • Implement listlike-argument decorator to modify a function expecting one Sequence-like type as input to flexibly accept star-args and other listlikes
  • Move allow_string_paths() and allow_pathlib_paths() into genutils.fileutils.pathutils (this is where I always think this lives anyways)