[preconditioners] state dune namespace in indices

Closed Dennis Gläser requested to merge fix/index-constant-ambiguity into master

I noticed that when using Dune-MovingMesh, there is a name clash in the Uzawa preconditioner, because mmesh is built on top of CGAL, which itself uses boost (if available only maybe). Boost also declares these indices leading to amibiguity here.

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