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[constraints] Fix contraint computation for hierarchical bases
The algorithm basically interpolates fine DOFs from coarse basis functions of neighboring elements. This requires some mechanism to interpolate only hanging fine DOFs and not all. Instead of checking if the fine interpolation point is in within the coarse element, we now check if the interpolated DOF is associated to a subentity of the intersection. In most cases this is equivalent. But for hierarchical bases, the former does probably not lead to the desired result (*). Remark: (*) In principle one would expect that the conforming hierarchical basis consists of the conforming P1-basis plus conforming bubbles. This is the case for both algorithms (except for some numerical issues for the old one). However, it turnes out that there are different possibilities to define the conforming bubbles and it's not clear which one is desired. Ideally we would like to have: * The bubbles are lagrangian, i.e. each one is zero at the interpolation point of the other ones. * The bubbles are non-negative. For hanging nodes you can't have both at once. The old algorithm guarantees the former, the new one the latter property. I'm undecides which one is 'correct'.
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