Fix a bug in which the switching function would not get toggled if requested via

the switchDistance argument to CharmmPsfFile.createSystem().

Also add a check to protect against negative switching distances.

wrappers/python/simtk/openmm/app/charmmpsffile.py

@@ -1185,6 +1185,9 @@ class CharmmPsfFile(object):
                 if switchDistance >= nonbondedCutoff:
                     raise ValueError('switchDistance is too large compared '
                                      'to the cutoff!')
+                if abs(switchDistance) != switchDistance:
+                    # Detects negatives for both Quantity and float
+                    raise ValueError('switchDistance must be non-negative!')
                 force.setUseSwitchingFunction(True)
                 force.setSwitchingDistance(switchDistance)
 
@@ -1226,6 +1229,9 @@ class CharmmPsfFile(object):
                 if switchDistance >= nonbondedCutoff:
                     raise ValueError('switchDistance is too large compared '
                                      'to the cutoff!')
+                if abs(switchDistance) != switchDistance:
+                    # Detects negatives for both Quantity and float
+                    raise ValueError('switchDistance must be non-negative!')
                 force.setUseSwitchingFunction(True)
                 force.setSwitchingDistance(switchDistance)