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.
