Longitudinal emittance in long_term_tracking

[jeu8]

For ease of use, the ability to initialize a beam using an emittance instead of bunch length and energy spread would be convenient.

[DavidSagan]

@jeu8 Is this always for closed lattice branches?

[jeu8]

I just realized I never replied to this. I would always be using this with closed lattices. Not use if anyone else would care about having this for open lattices