Large update:

    - Restore the 'old' API (atom.bonds, atom.angles, etc. is a list of Bond,
      Angle, etc. instances). bond_partners, angle_partners, dihedral_partners,
      etc. are once again the list of partners.

    - ChamberParm topologies now create an OpenMM System object that gives the
      same energies as those calculated by Amber and CHARMM. Accurate to within
      0.1 kcal/mol.