Create and name a subclass of RandomVariable to represent differenced timeseries

[dylanhmorris]

Motivation

Classes like FirstDifferenceARProcess below could be created by composing a representation of the differencing with a representation of the underlying timeseries of $n^\mathrm{th}$ differences.

https://github.com/CDCgov/multisignal-epi-inference/blob/99cdf4272358d80966ce92007d896245d53d59e2/model/src/pyrenew/process/firstdifferencear.py#L12-L17

Steps

• Choose a name for the class.
• Implement it.
• Reimplement the existing FirstDifferenceARProcess using it

[dylanhmorris]

Going to wait for #215 as it bears upon this.

[damonbayer]

Closed in #380