[Cbc] Adding more flexibility after cut generation

[John Forrest]

I will be adding some functionality to Cbc to examine generated cuts and 
prune them before adding them to model.

I will be doing this by adding a new event so that users can use 
CbcEventHandler to do this.  I would be grateful if adventurous users 
can give some input on what they might want - for the new event, 
extensions to existing events or new events.

Thanks

John Forrest