[!688] make `FieldMatrix` and `FieldVector` trivially copyable types

Merge branch 'FieldMatrix-and-FieldVector-trivially-copyable-types' into 'master'

ref:core/dune-common Closes: [#168], [#169]

See merge request [core/dune-common!688]

  [#168]: gitlab.dune-project.org/NoneNone/issues/168
  [#169]: gitlab.dune-project.org/NoneNone/issues/169
  [core/dune-common!688]: gitlab.dune-project.org/core/dune-common/merge_requests/688


Closes #169 and #168