Replace consistency checking with assertion

Replace consistency checking with assertion

The checks intended for checking that the automatic choice of number
of thread-MPI ranks have never worked, because we over-write the
user's choice of hw_opt->nthreads_tmpi. Fixed that by handling those
sub-cases properly by requiring that the number of GPU IDs supplied
matches the number of ranks chosen in all the cases, and asserting on
that.

Even if we would fix the over-writing issue, all of the former
consistency checks amount to an assertion that the previous code has
chosen a number of GPU tasks on each node (in gpu_opt.dev_use) that
matches the number of PP ranks on that node, which can be done in many
fewer lines of code.

Note that the checking did not have a SIMD-related behaviour, and
since the way domains communicate is SIMD-agnostic, it's unclear that
such a check would be useful.

Change-Id: Iea85bb4c297c5ce60c8e25d95a87fe9d9bce6686