More flexible Rt class (possibly meta)

[ghost]

Goal

Given that we now have a latent module for latent (but potentially observable) variables, I propose creating an an Rt class there.

To represnt varying in time according to a particular stochastic process on a particular transformed scale, one would instantiate the Rt class (or a subclass of it), using stochastic processes from pyrenew.processes, transformations from pyrenew.transform etc.
-- #26 (comment) @dylanhmorris

Context

Re-implementation of pyrenew.

Required features

See goals

Specifications

• Create the desired class under processes/
• Have a test.
• Have it properly documented, including an example in getting started or other doc under docs