Electric field mdp parsing using options

Electric field mdp parsing using options

Make the module declare a structured set of options, as well as a
transform from a flat mdp structure to this set of options.
Use these for assigning the input values, instead of passing t_inpfile
directly.

There is one regression introduced here: values not specified in the mdp
file are no longer automatically inserted with defaults, and comments
related to the electric field input values are lost.  With just the
electric field affected, this should not be a big deal, but that can
limit adoption of this approach until we can solve it (preferably, by
changing the mdp format to something where it is easier).

Change-Id: Ic71275f3533f06ce6fee64983c7e1686b782fb9b