DDM Tests: Rewritting SystemAbstract::getSolution + FunctionSpace::getUnorderedKeys() + FunctionSpace::getAllDofs with set + FunctionSpace::getKeys(set)

useful for remapping a mesh onto a (slightly modified) CAD, for sensitivity analyses

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In that case we search for all the matches in the octree, and we try to return amongst those the element that matches the input coordinates.

We use this in MathEval so we can reliably make operations on discontinuous datasets (e.g. edge element vector fields), on identical meshes that were possibly reordered.

Enforce volume positivity of 3D elements right after mesh generation/optimization, instead of enforcing when writing to file

Small simplifications + Preparation for Formulation Independent FS + Preparation for multiple Formulations using same FS