Re-implemented mdrun -rerun tests
Used the new infrastructure to let the rerun test check that the
energies and trajectories (including forces) produced by a rerun
actually match those produced by a normal mdrun. Such a test running
in parallel would have prevented #1868, and will help checking that
any changed or new integrator functionality does the job that it
should do.
Updated some infrastructure to not care whether it gets passed
a C or C++ string.
Removed small pieces of infrastructure originally intended for use by
this patch but which are no longer required.
Refs #1868, #2112
Change-Id: Ifcb211aba62ccb79ebaec21a3088c9e60a618c86