Create NullLatentModel

[SamuelBrand1]

          > I think I've nearly fully addressed this concern. Where we might be missing something is not having a NullLatentModel type which just generates nothing values. I wanted to leave the option open for the ODE models to be conditional on a latent process outcome, e.g. as per a Renewal model, but that is not currently used. A nothing returning process would let us use the current infra for model specification.

Good idea

Originally posted by @seabbs in #510 (comment)

As discussed here, this would be an easy and handy feature.

[SamuelBrand1]

It also makes resolving #526 smoother, because we can reuse the EpiProblem pattern.

[seabbs]

Yes good point