Free Space test cases + Nodal Interpolation + Small Fix in FunctionSpace: Bug with cell function in 2D for 3D Function Space + Vertices of GroupOfElement

tolerance cannot be choosen that large; in case of non-connected meshes we could end up interpolating too far outside the (wrong) element, leading to bad values

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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