Design of Particle and Path

[alicerobson]

At present the particle points to a path via a shared-ptr- This is not as clean as it could be. If the path gets changed then the particle is not logically const as one would like. NB At the moment the particle is physically const as the shared_ptr does not change, but this is not ideal.

Goal would be to allow particle to be const once created and keep path more separate.

One suggestion is to store the paths into a collection in the papas Event and for the paths to know about their particles. The particle may have to figure out which path is attached to it by looking at the history.