[fcstaggered] Use reference element for index mapping

Some grid managers only support subIndex for elements.
Using the reference element we obtain the vertex indices
via local index maps.
5 jobs for !2876 with feature/staggered-mapping-via-reference-element
latest detached
Status Name Job ID Coverage
  Trigger
passed full-dune-latest-release-clang

passed full-dune-latest-release-gcc

passed full-dune-master-gcc

passed minimal-dune-latest-release-gcc

 
  Downstream Modules
passed trigger lecture