Implement the DRW AGN model

[hombit]

Feature request

@qifengcheng and I created a damped random walk model for AGN variability based on the wavelength and parameter distributions we found in the literature. The up-to-date implementation is in the SNANA repo, here. It also includes a simple changing-state (changing-look) AGN behavior, which we can skip in the initial implementation.

The damped random walk is a simple continuous-time process, an astronomical name for the Ornstein-Uhlenbeck process with no drift, also equivalent to the continuous autoregressive process of the first order, CAR(1). The great thing is that it is the only continuous stochastic process (Doob42) that is 1) Gaussian (every observation over time is Gaussian distributed), 2) Markov (so you only need to know the previous observation to get a distribution for the next one), and 3) temporally homogeneous (properties do not change when shifted by an arbitrary time interval). And it also has a simple analytical solution, so it is pretty simple and fast to simulate.

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