Feature request: add integro-differential equation support

[sdwfrost]

There aren't many implementations of integro-differential equations which are useful for many things e.g. distributed delays, non-exponential distributions of passage times, time-since-infection epidemiological models (see here) and it would be great if MTK/the DiffEq ecosystem could support these.

[ChrisRackauckas]

Yeah, I think the best way to do this is to add it to the PDESystem spec so that way we can utilize it in physics-informed neural networks. @KirillZubov should be in tune with this.