Which finite element types are supported by the spectral analysis code?

[nfehn]

Does the spectral analysis code

https://github.com/exadg/exadg/blob/master/include/exadg/postprocessor/spectral_analysis/interpolation.h

only work for dealii::FE_DGQ, or would it also work for H(div)+RT?

If not, how should we extend this? Use dealii::RemotePointEvaluation?

In any case, we should make sure to assert that the finite element type is consistent with the underlying implementation of this functionality.

[kronbichler]

I think we should provide functions to interpolate the RT solution to a DG solution space (of the higher polynomial degree), and then use the interpolators. I think there are several places where a DG representation can be more straight-forward to use. Apart from the Piola transform on non-affine cells, this should be possible without changing the solution.