Cut plane

[termi-official]

Cut planes are essential for visualizing 3D stuff to inspect the interior of the domain. We want to add

1. crinkle plane
2. "clean" cut plane

for the latter we have to think about how to extract the plane efficiently in non-linear geometries

[termi-official]

So the algorithm for determining the clipping plane in elements is a bit harder, but I think I have something useful.

1. Distribute points on the reference element w.r.t. some measure of local (geometric) non-linearity
   Here my idea is to have some local AMR procedure, trying to approximate the solution with Lagrange{ref_dim,ref_shape,1} up to some tolerance.
2. Push forward from reference to physical space
   Standard procedure.
3. Determine which points belong to the boundary
   Standard clip plane algorithm on the linearized representation (which is now quasi-exact)
4. Generate triangulation of the boundary points
   Standard algorithm, again.

Since we only require refinement, the data strucutre should be easy to implement.