Heppy updates from CMG

Hello Colin, 

Here's all the updates to Heppy that we would like to make as part of porting CMGTools to Heppy in 7.2.X.
They include some improvements and new functionalities, and also bugfixes to things that had already been ported in Heppy by you or Andrea.
In the future we'll try to keep pull requests smaller, but in this case we wanted to first arrive at a point where we were able to reasonably run a CMGTools workflow.

@arizzi @mariadalfonso