Reduce the estimated degree only on simplex cells

[pbrubeck]

The problem

_reduce_degree() lowers the degree by one for each derivative. Its docstring
says that it does not do this when tensor product elements or quadrilateral
elements are involved. The code does not agree with the docstring. It tests the
name of the cell against quadrilateral and hexahedron only. A tensor product
cell has the name quadrilateral * interval, which is not in that list. UFL
therefore lowers the degree on a tensor product cell.

A derivative lowers the degree in one direction only. On a cell that is not a
simplex, the degree is a degree for each direction. The degree for each
direction therefore stays the same. To lower it gives a rule that is too coarse.

Prism cells and pyramid cells have the same problem.

The change

_reduce_degree() now lowers the degree only on a simplex cell. This is the
behaviour that the docstring describes.

Rules on prism cells and on pyramid cells become finer. Rules on simplex cells,
on quadrilateral cells and on hexahedron cells do not change.

Effect

Firedrake gives an element on a tensor product cell a degree for each direction.
The numbers below also need a change in FIAT, which makes
TensorProductElement.embedded_superdegree give the degree for each direction.
This pull request on its own does not change them.

Before that pair of changes, a Q7 space on a quadrilateral * interval cell got a
quadrature rule of degree 29, which has 3375 points. After this change, the same
space gets a rule of degree 17, which has 729 points. The rule with 729 points
integrates the form to machine precision.

form	points before	points after	flops before	flops after
Q7 inner(grad(u), grad(v))*dx	3375	729	33,516,981	15,941,187
NCE7 inner(curl(u), curl(v))*dx	3375	729	253,603,812	115,918,062

Each matrix agrees with a rule of degree 32 to machine precision.

Tests

test_tensor_product_degree_estimation puts a Lagrange element of degree 7 on a
quadrilateral * interval cell. It shows that a derivative does not lower the
degree. The test fails before this change, with a degree of 6 in place of 7.

AI declaration: written with Claude Code (Claude Opus 5).