Implement Parallel Blocks

[carldlaird]

It is possible to implement an MPI-based IndexedBlock that would allow creation of blocks in parallel - this would need to be paired with a solver interface that could exploit that and either perform decomposition or put the blocks together for a serial solve - not sure how much overall benefit this might provide.

[jsiirola]

Archived on the master Performance Proposals Issue (#1430). Closing this performance proposal until active development has begun.