This fixes several issues with how DIY was used in MultiBlock.

Instead of using `diy::RegularSwapPartners` using
`diy::RegularMergePartners` to reduce data to block(gid=0) and then
broadcast out to all ranks (and not blocks) using
`diy::RegularBroadcastPartners`. Old code that used RegularSwapPartners
ended up building reduced result on all blocks, which was not only
unnecessary, but expensive since we would generally have more blocks
than ranks.

Remove `DecomposerMultiBlock`. This class was needed due to my
misunderstanding of how the decomposer works.
`diy::RegularDecomposer<diy::DiscreteBounds>` provides all the necessary
functionality provided by `DecomposerMultiBlock`.