GPU accelerated iterative solver

[vboussange]

We should keep an eye on SciML/LinearSolve.jl#512. The AMGX solver could be huge in terms of speed gain.

[rafaqz]

There isn't anything to specifically do here besides try it (e.g. on the cleanup branch) and see where it breaks - LinearSolve.jl solvers will work but in practice something here wont work, or will be slow with GPU based arrays