semi-structured AMG for radiation solve

[BenWibking]

There is a new "semi-structured" AMG solver (SSAMG) in Hypre: https://arxiv.org/abs/2205.14273.

They claim it significantly reduces the setup time compared to the existing AMG solver, since it assumes logically-Cartesian mesh connectivity (possibly with AMR) using the SStruct interface, rather than the IJ interface, while being as robust as AMG. I don't know if there's a GPU version yet.

A comparison paper between implicit FLD with this method and our method in Quokka would be potentially interesting.

[BenWibking]

For reference: hypre-space/hypre#888. The SSAMG code appears on be on this feature branch: https://github.com/hypre-space/hypre/tree/recmat.