Add rare processes to default cross section list

[Jean1995]

This discussion has been brought up at several developer meetings. At the moment, the default CrossSections (listed here) only include interactions which we have declared as "standard".

This does not include:

• Muon pair production by muons (mu -> 3 * mu)
• Muon pair production by photons (gamma -> mu mu)
• Weak interaction of charged leptons

Why should they not be included within the default cross sections if our goal is to provide the most accurate simulation result possible? At least for the first two processes, I can directly see the relevance for several users of PROPOSAL.

An argument "against" the inclusion of these cross sections in the default would be the additional time to create the cross section tables, and maybe a slightly worse performance (although this should only be very small effect).

[sudojan]

Indeed the additional storage for interpolations as well as the worse performance were some arguments not to include them in the default. But also that all other simulation tools propagating HE muons use just these 4 interactions as default, so users would experience unexpected behavior. However, as all things evolve and the additional overhead is relatively small, I have nothing against PROPOSAL setting a new/improved default regarding HE muon simulation.