Add support for dynamic container of local solvers

This allows to have different local solvers in `NonlinearGSStep`.
The feature existed before but was restricted to statically sized
containers. This patch adds support for dynamically sized ones.
4 jobs for feature/nonlinear-gs-dynamic-localsolver in 8 minutes and 13 seconds (queued for 7 seconds)
Status Name Job ID Coverage
  Test
passed dune:git clang C++17 #176763

00:05:40

passed dune:git clang C++17 with examples #176765

00:08:12

passed dune:git gcc-8 C++17 #176764

00:07:39

passed dune:git gcc-8 C++17 with examples #176766

00:02:51