Convert truncateVector/-Matrix into free functions

This way, they can more easily be used in other contexts.
2 jobs for feature/truncate_as_free_function in 7 minutes and 10 seconds (queued for 1 second)
latest
Status Name Job ID Coverage
  Test
passed dune:git clang C++17 #99454

00:04:31

passed dune:git gcc-8 C++17 #99455

00:07:10