A Cartesian topology can be partitioned into a set of Cartesian
topologies of lower dimension using;
mpi_cart_sub (comm, remaindims, newcomm, ierr)
remaindims[i] is true if the ith dimension is retained, and is
false if it is discarded.
Example:
If a 
Cartesian topology is associated with
comm, and
remaindims = (true, true, false)
then the above call to mpi_cart_sub
- creates 4 subcommunicators, each with a
Cartesian
topology,
- the value of newcomm returned is that containing the
calling process in its associated group.
