Fix embedded elements

[termi-official]

Embedded elements are currently broken in several ways and require several workarounds to function properly.

First workaround to make the constraint elimination work is the assumption that "faces = edges" and "cell = face" for 2D elements in 3D. The dof handlers already work without this assumption, however the constraint handler still requires this. The new hotfix for this has been introduced here https://github.com/Ferrite-FEM/Ferrite.jl/pull/581/files#diff-3e76902e9a74000d5b26953f9acefca30977e4844e04df825a59ebe4ab77469eR43-R47 . Embedded 1D elements in 3D are still fully broken. This issue has been around for a while and is related to #394 .

The next point of failure is in the Cell*Values, where the transformation between reference element and physical space is not implemented. The linear shell example has a hotfix, but this hotfix just works for the specific formulation. Here it might make sense to have a dispatchable function which computes the transformations (and weights) correctly for the specific formulation. It should be noted that this might need changes in Tensors.jl (see Ferrite-FEM/Tensors.jl#188 ) but other approaches should also be feasible.