Updated grompp cut-off and PME-Switch checks

Added a warning with PME-Switch when switching over more than 5%.
Charge-group size rlist notes/warnings now also appear when
potential modifiers are used.
Documented effect of potential-modifiers on the group scheme cut-off
treatment in mdp_opt.html.

Fixes #1179

Change-Id: Ide00caf5ae378104f99611bf197e896d1b53d7d7