Add Gen2 classification efficiency

[jvansanten]

The cascade/double-cascade/track classification efficiency used in the Gen2 TDR never made it into the repo. Add it now.

This is motivated by differences that @colemanalan observed between toise-plot diffuse.flavor.muon_damping and Figure 25 of the Gen2 TDR, made with https://github.com/icecube/toise/tree/c7a63cd78dc9148f498d0f0946cf98471070713d. While most of this difference is attributable to the updated classification efficiency, some differences remain due to the different treatment of the surface veto:

• The TDR plot assumed that the surface veto removes all penetrating atmospheric muons above 100 TeV from the throughgoing muon sample that pass through the footprint of a 10 km^2 surface array. The current treatment uses a realistic parameterization of the surface veto efficiency, and assumes a 5.8 km^2 array.
• The TDR plot assumed that the combination of surface and outer-layer vetoes would reduce the downgoing atmospheric neutrino background by a factor 1e-4. The current treatment is conservative, assuming no reduction at all.