Message warning about suboptimal performance of AMG in case no direct solver is available
If no direct solver (UMFPack, SuperLU) is found (both are optional dependencies),
the coarse grid solver in the AMG is a BiCGSTABSolver hard-coded to a tolerance of 1e-2 (see https://gitlab.dune-project.org/core/dune-istl/-/blob/master/dune/istl/paamg/amg.hh#L771) which
may not be precise enough depending on the application.
I suggest to print a warning if an AMG-based solver is instantiated and neither UMFPack or SuperLU is found.
We have a patch in dumux that can be applied to decrease the tolerance (https://git.iws.uni-stuttgart.de/dumux-repositories/dumux/-/blob/master/patches/istl-2.6.patch)