[!452] Add pivoting switch to DenseMatrix

Merge branch 'pivoting_switch' into 'master'

ref:core/dune-common As discussed in [!449], the user should be able to
disable pivoting if needed. This merge request is intended as a basis for
discussion and adds a simple static switch to DenseMatrix to disable pivoting.
Depends on [!449] and [!451], the only relevant commits for the switch are the
last two.

See merge request [!452]

  [!449]: gitlab.dune-project.org/NoneNone/merge_requests/449
  [!451]: gitlab.dune-project.org/NoneNone/merge_requests/451
  [!452]: gitlab.dune-project.org/core/dune-common/merge_requests/452