Muons

Added information to full chain for the cosmic ray muon background using an approximation from IC86 Corsika files and Muon Gun weights
This also might be solved nicer by defining a dict further up
```
scales = {flavour:nu_xsec_scale for flavour in flavours if not flavour.contains('bar')}
scales.update({flavour:nubar_xsec_scale for flavour in flavours if flavour.contains('bar')})
scales['muons'] = muon_scale
```
and then further down just use `livetime*scales[flavour]` for everything.