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().
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