Add more square root kernels

[gw265981]

What's the new feature?

The Kernel Thinning algorithm (https://arxiv.org/pdf/2105.05842) recommends using square root kernels. #867 added a square root kernel to SquaredExponentialKernel, but adding this to other implemented kernels can require a lot of effort.

What value does this add?

According the Kernel Thinning paper, using square root kernels in the KT-split stage is optimal theoretically. It is difficult to quantify how much this really matters in practice.

Is there an alternative you've considered?

No response

Additional context

No response

[tp832944]

This should be subdivided into one ticket per kernel to add.