stopTracks option has double function

[Jochem Snuverink]

the stopTracks option currently has a double function:

• kill tracks after energy has been deposited (after e.g. scattering)
• kill secondaries (except for muons)

It would be good to split this up and create an additional separate option.

[Jochem Snuverink]

fix issue #45 : add option stopSecondaries (kills all secondaries)

→ <<cset 01e035babb78>>