ENH: Adjust exodus writer to utilize material blocks.
Previously, when converting from xdmf to exodus, spatial partitioning (metis) in xdmf was written as element blocks in exodus. However, exodus uses element blocks to specify different materials and not as spatial partitioning for parallel execution. We now write materials to blocks in exodus, indicated by Block node sets in xdmf. This now makes the conversion of xdmf -> exodus the exact inverse of the exodus -> xdmf function.