Generic tpr handling for electric field

Generic tpr handling for electric field

Remove special code to store inputrec values to tpr files from
IInputRecExtension, and instead store them as a generic KeyValueTree.

Currently, there is no support for changing the storage format, other
than by adding new options. We may not need this immediately, and we
think it should not be any extra work to introduce it later (rather
than right now). With the present approach, tpr backwards
compatibility is essentially the same as mdp backwards compatibility.

There are now two paths to setting up modules for use, depending
whether the data is coming from mdp parsing in e.g. grompp or tests,
or from tpr parsing in e.g. mdrun, dump or check (but not tpbconv).

Change-Id: I0a0f552041b2a7401bc82972c9648d1f3a64af34