Prepare Verlet scheme later in runner
As noted in the TODO, preparing the Verlet scheme can wait until we
need rlist chosen so that we can set up the domain
decomposition. Moving it later in mdrun helps clarify the unrelated
process of setting up thread-MPI, and also means that the state used
when preparing the Verlet scheme is the one from the checkpoint file,
where applicable.
Moved a warning for the group scheme on BG/Q to a more appropriate
place.
Noted some TODOs for managing kinds of state better in future.
Change-Id: I99a014d9465b46b32038cfdcee99dc3896d2f265