The block-Jacobi partitioner (commit e360c00b7) uses grid interfaces
that are only available in CpGrid and this blocks introducing other
grid managers such as ALUGrid. Since the partitioner was only added
for OpenCL, guard special purposes accesses with HAVE_OPENCL.
This is a temporary measure and we will venture to restore the
partitioner later, although possibly restricted to CpGrid only.
Mostly to reduce the number of nested scopes in doLoadBalance_() and
to make the data flow a little easier to track for human readers.
While here, also use down-casts to pointer instead of references.
Doing so replaces try/catch blocks with simpler if/else blocks.
Reimplement face transmissibility calculation/extraction loop in
terms of helper functions elements() and intersections().
This commit broadcasts the 'removed_cells' to all ranks and uses
that array to delete any analytic aquifer connections that might
have been previously formed to those deactivated cells. This is
needed to have a consistent view of which cells are connected to
analytic aquifers.
