feat: Vecchia / nearest neighbour approximation kernel

[daniel-dodd]

Feature Request

Introduce Vecchia approximation that is popular in spatial statistics https://en.wikipedia.org/wiki/Vecchia_approximation

Like our RFF kernel, have a Vecchia kernel with a Vecchia compute engine to do the approximation. Then have a notebook of a spatial / geostatistical problem, contrast with a sparse variational GP.