Implement new lattice points on simplex cells

[mscroggs]

On simplices, GLL points lead to elements with a lower Lebesgue constant than equispaced points, but the Lebesgue constant still grow rapidly the polynomial order is increased:

The Lebesgue constant for elements generated with Fekete points will be better behaved, although it seems that an optimisation problem needs to be solved to find these, and they are often not symmetric. A set of (preferably symmetric) points leading to elements with a good enough Lebesgue constant are desired.

[wence-]

Toby Isaac has a simple symmetric construction with good properties for simplex nodes of all dimensions in this preprint https://arxiv.org/abs/2002.09421

[mscroggs]

Toby Isaac has a simple symmetric construction with good properties for simplex nodes of all dimensions in this preprint https://arxiv.org/abs/2002.09421

Thanks, this saves me a lot of searching and reading