[Dune] vote: what should the codim<3> iterator do in the curvilinear case

[Aleksejs Fomins]

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Dear Dune,

I just realized a slightly puzzling thing:

In curvilinear geometry, an element has a set of interpolatory
vertices. For a tetrahedron, there are always 4 corners, plus a bunch
of internal vertices based on interpolation order of the element.

Currently, I have implemented it in the grid such that all
interpolatory vertices are codim<3> entities. This means that
currently, if one iterates over codim<3> entities, the interpolatory
vertices will also participate.

Having said that, I personally think it does not make sense. If
somebody wishes to implement a nodal-based FEM code, they probably
only care about the corners. I do not see a reason why should a user
ever care about the non-corner interpolatory vertices, unless he
directly accesses the Geometry class.

My proposal:
1) All vertices are codim<3> entities
2) standard codim<3> iterator will only iterate over corners
3) an additional InterpolatoryIterator will be provided which will be
able to iterate over all codim<3> entities
4) DataHandle communication interface will only explicitly be allowed
to communicate over corners.

What do people think about this approach

Best,
Aleksejs
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