Block Gauss--Seidel steps and related tests

  • deprecate current redundant solvers
  • test non-scalar block problems where local ignore nodes are not either all true or all false
  • test that ignore nodes are respected.
  • currently the ProjectedBlockGS is tested, but the obstacles are not relevant to the exact solution
  • provide implementation for cachedldlt local solver (necessary?)
  • extend the framework to support nonlinear steps, e.g. ::ProjectedBlockGS (overlap with dune-tnnmg/feature/new_tnnmg)
  • allow more deeply nested blocks (and ignore nodes)
  • extend recursively, in analogy to the ::TruncatedBlockGS class
  • provide Gauss--Seidel loop that accounts for a SumOperator with LowRankFactor (eventually enables fully replacing ::TruncatedBlockGSStep)
  • test TrustRegionGSStep (in an appropriate setting, i.e. with non-convex block problems)
  • provide some mechanism or at least satisfying documentation to warn about local solvers eventually not generating a symmetric linear iteration step despite the outer Gauss--Seidel loop being symmetric (i.e. forward and backward)
  • provide a test that assures that non-existing (diagonal) blocks yields the same result as zero (diagonal) blocks.
  • provide checks (or at least documentation) that semidefinite diagonal blocks impose assumptions on the sanity of the rhs.
  • make use of row iterators (maybe using a range-based for wrapper to be provided to dune-common or dune-matrix-vector soon)
  • allow for generic ignore nodes implementations (out-source the usage of any() and alike because the ignore nodes type might not provide these methods)
  • remove support for an ignore node set that is not set (i.e. add existence checks before the actual solve and do not create temporary local ignore nodes)