Create `numerics_t` type
Create-numerics-type on the parts of inference_engine_t that are needed for inference
@ktras please ignore this comment. This is a demonstration.
@ktras I'm forgetting the motivation for this PR. If you still want to merge it, we can discuss it the next time we talk and then I can review it.
@ktras this has suffered some bit rot due to name changes in the relevant code. I suspect this might not be needed anymore so I'm closing it. If you think it's still useful, feel free to make analogous changes on develop. If so, then you probably want to wait until I merge PR #218, which implements a lot more renaming.