Warn if using nsteps=0 without continuation=yes

To achieve true single-point energy evaluations for a structure
we should not constrain the initial conformation. This is
typically done by settings nsteps=0, so grompp now warns if this
is used while keeping continuation = no (the default).

Fixes #1231.

Change-Id: I4e65b66f6535da4206e7dbd5f7769a33c94ec12d