Store mgStep in a non-smart pointer

The interface of Solvers::IterativeSolver used to make its
iterationStep_ member publicly available.  This has changed,
and the iteration step can now only be accessed by a getter
method (which is good).  However, that getter returns a
reference, but not the full smart pointer.

See the discussion at

  https://git.imp.fu-berlin.de/agnumpde/dune-solvers/merge_requests/23