Avoid near-silent generation of zero energies

Avoid near-silent generation of zero energies

There is note printed about this case for Verlet+GPU+energy groups,
and this is OK for normal simulations because the decomposition of the
energy into groups is unimportant. However, a user doing such a rerun
is probably seeking the energy decomposition, and in that case getting
zeroes is unexpected behaviour.

Instead, default to not using GPUs with mdrun -rerun -nb auto and more
than one energy group. In that case with mdrun -rerun -nb gpu, give a
fatal error and suggest the probable solutions.

Change-Id: I3006ed3ded31620d4faf4fc4a24cc85e9322c5c2