quasiseparable kernel convolution

[bmorris3]

Hi @dfm et al.,

tinygp is solving lots of my problems, thanks for your hard work.

One feature that's in celerite2 but not tinygp is TermConvolution (source), which convolves a quasiseparable kernel with an observation duration.

What would the path look like to implement its equivalent in tinygp? I'd be happy to make a PR if I had some pointers on where to start.

Thanks!