Add multiple scattering process

Focus on muons.

mentioned in issue #77 (closed)

mentioned in issue #108 (closed)

added Critical label

changed milestone to %ICRC2019

changed milestone to %Full physics demonstratror

@rulrich Can you provide some more info on this issue? (in case someone wants to implement it!)

Let's close this. There is no obvious path to action here. Multiple scattering is basically a sub-feature of PROPOSAL or other e.m. models, and is actually there already. Not with all aspects, but this issue here does not help with that...

closed