Implemented grompp -po for options-style mdp handling

Implemented grompp -po for options-style mdp handling

Previously options that were handled via a key-value tree (currently
only electric field options) were not written to the .mdp output.
Restoring this functionality will permit more widespread use of the
key-value tree handling during the transition period.

Restored mdp file comments for electric-field options, but the method
for writing them is hacky. This is probably good enough for
functionality that will disappear when we shift to key-value input.

Change-Id: I488b37ff72f70a6e145338fd9d57daa1c9e4de7e