The AMG within dune-istl issues a warning if ParMETIS is not available.
The reason is that it uses ParMETIS or the ParMETIS bindings of PT-Scotch
to redistribute the load on the coarse levels. Even with an installed and
found PT-Scotch we need to adapt the include path and the libraries linked
to make it work.
Therefore this commit adds a module for searching for ParMETIS and activates
it whenever dune-istl is a dependency. With this the warning about missing
ParMETIS for flow_cp vanishes.
