sea ice partitioning tool needs to take an MPI exec and core count

[xylar]

The sea-ice partitioning tool currently calls ESMF_RegridWeightGen but it doesn't have a way to call it with a parallel executable or with a number of cores. On some machines, even a 1-core call has to be made with a parallel executable. And it would be nice to use multiple cores if available.

For task parallelism, it may also be necessary to break those calls out into their own separate steps, but that can be left for future work.