theorashid

## Description This builds on @aseyboldt's work in #944. It is part of the INLA hackathon (https://github.com/pymc-devs/pymc-experimental/issues/340, specific related issues https://github.com/pymc-devs/pymc-experimental/issues/342 and https://github.com/pymc-devs/pymc-experimental/issues/343) and will later involve rewrites for the...