Muons

Added information to full chain for the cosmic ray muon background using an approximation from IC86 Corsika files and Muon Gun weights
Instead of this and the lines below, I think it would be nicer to use a `dict` for `{prim: service}` like this
```
services = dict.fromkeys(primaries,nu_flux_service)
services['muons'] = muon_flux_service

for prim, service in services.items():
    maps[prim] = {'ebins': ebins,
                           'czbins': czbins,
                           'map': service.get_flux(ebins,czbins,prim)}
```
This will give us more flexbility in the future, e.g. when we add a `NoiseFluxService` or whatnot. Also can you
   * change `flux_service` to `nu_flux_service`
   * add `prim` as argument  to `MuonFluxService.get_flux()` and `raise ValueError` inside `if prim is not "muon": `