Factory for creating t_inputrec

Factory for creating t_inputrec

Add gmx::MDModules that encapsulates initialization of t_inputrec in all
places where it gets created.  This provides a single place that needs
to change with the introduction of a more modular approach for mdrun
modules.  Child changes will demonstrate how this can be used for the
external electric field code.  Split into a separate change to keep the
main change smaller and more focused.

For now, put the code in mdrunutility/, as it needs to be quite high in
the dependency stack, but a better home could be found in the future.

Related to #1972.

Change-Id: I0d86fe1a8bb8f8f872f5b37bc4e7931dceae9f09