Make dune-parmg an optional dependency of dune-gfe

dune-gfe contains a bit of home-grown infrastructure for distributed
assembly of the tangent stiffness matrix.  This will now be removed
in favour of dune-parmg code, for better maintainability.  Also,
eventually we want the parallel solver of dune-parmg in addition to
the parallel assembly.

The new dependency is optional, to cater for people without interest
in distributed computing.

dune.module

@@ -8,4 +8,4 @@ Version: svn
 Maintainer: oliver.sander@tu-dresden.de
 #depending on
 Depends: dune-grid dune-uggrid dune-istl dune-localfunctions dune-functions dune-solvers dune-fufem dune-elasticity
-Suggests: dune-foamgrid
+Suggests: dune-foamgrid dune-parmg