This MR adds Ginkgo (https://github.com/ginkgo-project/ginkgo) as a new back-end for linear solvers.
First list of To-dos:
-
run solver on GPU -
move some implementation details to ginkgo (dirichlet bc handling) -
performance comparison with Petsc -
add MINRes solver to ginkgo -
support parallel assembly (node-level) in ginkgo