[!237] Feature/modified FCGSolver and CompleteFCGSolver

Merge branch 'feature/completeFCGSolver' into 'master'

ref:core/dune-istl I have modified the FCGSolver a little bit so that it's
easier to handle, and also renamed it for clarity to RestartedFCGSolver. Than
I derived the CompleteFCGSolver from it.

The CompleteFCGSolver uses as many old search directions as possible. This
results in more stability, but also in more average memory consumption.

See merge request [!237]

  [!237]: gitlab.dune-project.org/core/dune-istl/merge_requests/237