fixed issues with FEP soft-core and cut-off's

Now the cut-off is applied to the PME mesh correction for perturbed
pairs with soft-core. Now the cut-off is applied to VdW with soft-core.
Fixes possible NaN in free energy kernel with r close to 0.
Replaced a confusing grompp warning with PME and soft-core by a note.
Removed an invalid grompp warning about soft-core and twin-range.
Fixes #1146

Change-Id: I79a06b20158df2bce575808b6e31e660163bd307