Made shells work with the Verlet scheme

The issue was that atoms iso charge group should be put in the box.
Additionally water_pol only worked within a charge group, now fixed.
Note that with shells working across charge groups, which is always
the case with the Verlet scheme, no shell prediction is done.
We should implement prediction, since it improves performance.

Fixes #1429.

Change-Id: I2ebfc2d91fc161167f8f2573b61e1f519cf11fd8