Remove redundant total_ordering decorator usage

[DanielYang59]

Summary

• Remove redundant total_ordering decorator usage, as total_ordering is already applied on superclasses and child classes did not redefine any rich comparison operations.
• Clean up comment of equality definition of core.IStructure
• Minor import clean up (avoid from io import StringIO when it's used only twice as io is already imported)
• Minor docstring clean up

[DanielYang59]

I guess total_ordering was applied on IStructure at some point but I believe this comment should be removed now (currently it more servers as an equality definition):

pymatgen/src/pymatgen/core/structure.py

Lines 1006 to 1007 in 31f1e1f

	class IStructure(SiteCollection, MSONable):

Also current implementation of IStructure doesn't fulfill the prerequisite of applying total_ordering (only __eq__ is defined):

The class must define one of lt(), le(), gt(), or ge(). In addition, the class should supply an eq() method.