Implemented fatal error for nstcalcenergy!=1 with shells.

The combination of using shell particles with a value for
nstcalcenergy that is different from 1 leads to energies
not being communicated around on a parallel machine and hence
a meaningless shell relaxation procedure. This patch will just
bail out with a fatal error if the combination is detected.
Fixes #1376.

Change-Id: Ia54f1480b2358355438acb5de31dfa56b7f8d603