Draft: Ginkgo Integration

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