[BUG] ParmEd box vectors are occasionally incompatible with OpenMM

[lohedges]

Occasionally triclinic box vectors generated by parmed (having read box dimensions and angles from AMBER rst7 format) are incompatible with openmm, which raises the following exception:

Traceback (most recent call last):
  File "test1/run.py", line 146, in <module>
  File "test1/run.py", line 56, in test_openmm
  File ".../openmm/openmm.py", line 2292, in setPeriodicBoxVectors
    return _openmm.Context_setPeriodicBoxVectors(self, a, b, c)
openmm.OpenMMException: Periodic box vectors must be in reduced form.

This can be avoided by passing the parmed box vectors through the internal openmm.app.internal.unitcell.reducePeriodicBoxVectors function before passing them through to the context.