Add all gather to mpas_dmpar

[jonwoodring]

This would be useful to have to coalesce eddy connected component data in the ocean Okubo-Weiss analysis member.

The cross processor communication is currently implemented with a dmpar max reduce to gather the eddy statistics, but it would be cleaner with an all gather. This is because with a max reduce, the input array has to be as big as the output array, and properly initialized to negative values to get correct values after the max reduce. With an allgather, each processor just has to have a segment of the final array which is then joined together with the gather.

[mark-petersen]

This issue has gone stale. Closing.