dune-parmg is an optional dependency of dune-elasticity now

As a first step towards a full parallel solver for finite-strain
elasticity problems.

dune.module

@@ -8,4 +8,4 @@ Version: 2.5
 Maintainer: oliver.sander@tu-dresden.de, youett@mi.fu-berlin.de
 #depending on
 Depends:dune-common dune-grid dune-istl dune-solvers dune-fufem dune-geometry dune-functions
-Suggests: dune-uggrid
+Suggests: dune-uggrid dune-parmg