Fix typo in ndof attribute
Fix typo in ndof attribute

In `WaterBox` and `WaterCluster`, the `ndof` attribute was computed incorrectly when `constrained=True` -- only 3 d.o.f. were subtracted from the system, regardless of the number of waters in the system.

Fix: subtract 3 d.o.f. per water.
choderalab/openmmtools,choderalab/openmmtools