The multigrid solver regularly "rebalances" arrays, by which
it takes an input array and moves complete rows (i fixed, j
variable) such that the same global arrays divides
differently.  This is used on initial calling (to adjust the
input problem specification), on coarsening, and finally to
collapse "down" processors when near the coarsest level.

However, this code was written naïvely: it built (and
divided) the global array split at each invocation, using a
number of MPI_Allgather calls on each processor.  Profiling
(courtesy Kris Rowe) has found that this can be agonizingly
slow; in at least one test case on one particular system it
consumed over 50% of wallclock-time.

This patch fixes the problem by allowing rebalance_array
(now moved into the multigrid solver class) to cache these
array divisions.  It requires the caller to specify one of a
few categories, with the division being computed (with
Allgathers) only on the first invocation per multigrid
level and type.  The divisions currently are:

FFine, UFine, FCoarse, and UCoarse

where 'F' and 'U' refer to the error and solution terms
respectively.  Confusingly, coefficients of the Laplacian
itself (such as the uxx term) belong to the 'error' space in
terms of call structure.

This caching eliminates all but setup calls to Allgather.