Port TrustRegionSolver to current dune-fufem

Sander, Oliverrequested to merge
port-to-newer-fufem into master

Parts of the TrustRegionSolver code still relied on the old basis interface of dune-fufem. That is now being removed from dune-fufem, and this patch does (some of) the corresponding cleanup in dune-elasticity.

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