enabled DUNE-ISTL::AMG and DUNE-ISTL::CGSolver.
The default is the same as before, i.e. DUNE-ISTL::ILU0 and DUNE-ISTL::BiCGSolver
for solving the elliptic problem.
Merge remote-tracking branch 'upstream/master'
apply nitpicks comments and avoid delete.
Merge pull request #199 from dr-robertk/master

enabled DUNE-ISTL::AMG and DUNE-ISTL::CGSolver in CPRPreconditioner.