Add non-parametric reference date model about epinowcast

[seabbs]

A simple way to do this is to only allow non-parametric hazards to be set across all reference dates. This would mean having a design matrix with rows equal to the maximum allowed delay and arbitary feature. This vector of delays could then be exposed to the user to enable them to build models. The features exposed would need to be those that do not vary with time.

This formulation of the model, whilst limited, is comparable to what others have implemented and use. It may be possible to extend it in the future with additional complexity.

[seabbs]

I've planned out how to implement this feature in branch seabbs/issue4 with a view to implementing a prototype in the next few months.