[el1p2c] Optional use of dune-pdelab + amg or SuperLU, now working if only one...

[el1p2c] Optional use of dune-pdelab + amg or SuperLU, now working if only one option is available, and message if both are missing.

Reviewed by alexk.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@12851 2fb0f335-1f38-0410-981e-8018bf24f1b0