This commit adds support for loading a three-column NLDD
partitioning scheme of the form
MPI_Rank Cartesian_Index NLDD_Domain_ID
from a text file. In an MPI run it is assumed that the first column
holds integers in the range 0..MPI_Size()-1, and typically that each
such integer is listed at least once. In a sequential run, the MPI
rank column is ignored.
With this scheme we can load the same partition files that we write,
for increased repeatability and determinism, and we can also
experiment with externally generated NLDD partitions.
This commit switches the current implementation of
'partitionCellsZoltan()', i.e., 'partitionCells("zoltan", ...)' into
using the MPI-aware ParallelNLDDPartitioningZoltan utility. In
doing so we make 'partitionCellsZoltan()' private since its
availability is not guaranteed. We also slightly reorder the
parameters and switch from passing a "Grid" into passing a
"GridView" as an argument to partitionCells(), and specialise this
function for the known grid views in OPM Flow.
We extract the Zoltan-related parameters out to an Entity-dependent
helper structure and move the complexity of forming this type to a
new helper function, BlackoilModelEbosNldd::partitionCells().
