Change in particle id scheme for atm. muons

There seems to have been a change in the way the atm. muon MC saves the particles. Now the primary is first saved in the list as 81, a pseudo particle. Next follow the 13's, the muons. This is probably only important for the neutrino chain, as it involves particle identification. I propose to hardcode the particle_type to 13, as there is anyway a dedicated extractor function for muons in the neutrino chain.