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The pickle round trip for the Bond object does not preserve the `type` and `order` parameters in Python 2.7.

Near as I can tell, its because the parent `namedtuple` already has a __getstate__ method, so the default
of returning the `__dict__` is never given, so `__setstate__` is not called (as default). This leads to pickle
 restoring the state, but never invoking the `__new__` statement, resulting in `type` and `order` being
 lost.

 This PR fixes this problem in python 2.7, without breaking python 3.x versions.

 Related mdtraj/mdtraj#1308

Remove optimization to ForceField LennardJonesGenerator that was causing issues with CHARMM conversion

There are times when it is helpful to have the energy terms actually
added to the System even if that degree of freedom is constrained. Other
parsers (notably the Amber parsers) already support this keyword, so add
that support to ForceField as well

This also replaces some string-based element detection algorithms with
whatever the PDB parser does (by comparing directly to atom.element)

createSystem for both Amber and CHARMM files.

take everything out to a function, RBTorsion and CustomTorsion now use ordering=charmm as their default