AR(1) latent process

[seabbs]

As part of #3 we need an AR(1) process to be implemented. Ideally, this would also be general enough to support differencing and potentially AR(p) processes.

Potential inspiration (doesn't look possible to depend directly?):

• https://github.com/LAMPSPUC/StateSpaceModels.jl/blob/master/src/models/sarima.jl

[SamuelBrand1]

Thinking about how to specify this mathematically:

n differenced AR(p) process:

\Delta^{(n)} X_t = \sum_{k = 1}^p \varphi_k X_{t-k}

[SamuelBrand1]

Hows this going?

[seabbs]

All good just being slow. Should have a PR today.