setting -pc_hypre_boomeramg_relax_type_coarse

[Nico Schlömer]

I'd like to set the PETSc HYPRE parameter "-pc_hypre_boomeramg_relax_type_coarse", but unfortunately it doesn't appear in the list of settable options yet.
Please add

      if (parameters("hypre")("BoomerAMG")["relax_type_coarse"].is_set())
      {
        const std::string type = parameters("hypre")("BoomerAMG")["relax_type_coarse"];
        PetscOptionsSetValue("-pc_hypre_boomeramg_relax_type_coarse", type.c_str());
      }

to the list in PETScPreconditioner.cpp.

PS: I'm not quite sure as to why the list of settable options is restricted the way it is. It would seem a lot easier to me to just pass on every key-value pair to into PETSc and have PETSc worry about verification of the parameters.