Added warning for unnecessary soft-core

The sc-coul mdp option is, presently, only active with lambda states.
grompp now issues a warning when using soft-core without Van der Waals
decoupling without lambda states.
Also fixed an incorrect twin-range grompp check with PME.

Refs #1421.

Change-Id: I0605fe0f735d69f96f478612a00434eccef6232f