Add a mol2 reporter for OpenMM

[davidlmobley]

Often, I want to save structures of molecules from a simulation (i.e. gas phase) to a format which retains the charges, bond orders, and coordinates. None of the current OpenMM reporters can retain all of this info since OpenMM doesn't retain bond orders.

I should make a mol2 reporter which is initialized with an OEMol or OEMols for the components of the system, and then writes coordinates from a simulation/minimization to .mol2 format when done.

For my own reference, an easy entry point would likely be to consult the OpenMM PDB reporter:

• class structure for reporter: https://github.com/pandegroup/openmm/blob/master/wrappers/python/simtk/openmm/app/pdbreporter.py
• file definition: https://github.com/pandegroup/openmm/blob/master/wrappers/python/simtk/openmm/app/pdbfile.py

[davidlmobley]

See also #167