Further improve getDDGridSetup

Further improve getDDGridSetup

Merged and simplified two similar setup functions that were sharing
the setting up of the number of PME-only ranks and the DD grid used on
the PP ranks. No functionality changes here, although some user
messages are slightly improved.

This helps create the seam that GPU halo exchange setup will need in
order to do a good job of triggering that communication path, namely
that there is a clear point when the number of PP ranks is known, so
that the search for a DD cell decomposition can be made in a way that
might suit GPU halo exchange.

It also permits more of the DD setup routines to be called without
leading to a fatal error or output confusing to a user, which will also
be needed when introducing support for GPU halo exchange.

Functions called in init_domain_decomposition no longer look up the
total number of ranks from cr, but instead accept an integer
parameter. This will enable thread-MPI checking for various
possibilities to work smoothly if we ever do that.

Used better variable naming, IVec and more const
correctness to improve readability, and improved comments.

Noted TODOs for further work at some future time.

Improved havePPDomainDecomposition (and some of its users) so
that we are less likely to erroneously use it before we have
initialized DD.

Change-Id: I65e285cc10d946c7c5bf298d7ff08c81ba05f3d8