More constexpr!

Santiago Ospina De Los Ríosrequested to merge
feature/more-constexpr into master

This MR focuses in FieldVector, FieldMatrix and iterator utilities. In particular, this is a step towards a constexpr reference elements in dune-geometry.

Note that this needs !1388 (closed) to be resolved.

Edited by Santiago Ospina De Los Ríos

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