Open requested to merge coarse_solver_from_solverfactory into master
This MR enables the creation of the coarse solver in AMG from a
ParameterTree using the solver factory. As mentioned in #81.
I tried to mimic the (not trivial) behavior that is currently implemented if no coarse solver config is given.
Furthermore it fixes #62.
There are a few issues:
This MR depends on !421 (merged) (should be merged before)
The solver factory does not support parallel operators, even if only on process is involved (addressed by !428 (closed)!453 )
The direct solvers do not support vector types with an allocator different from the allocator of the matrix. This is why the CI fails currently. (addressed by !423)