Initialise simulations with <epichains> objects

[jamesmbaazam]

We are considering the ability to initialize new transmission chain simulations with simulated <epichains> objects. This would allow users to chain together sequential simulations where previous chains are altered before being passed to the next.

A use case is to model interventions based on conditions such as the generation or infection time. Users could for example prune previous chains before passing to another simulation.