[WIP] Total efficiency calculation

I added two methods to the Efficiency class to compute the total efficiency (weighted by the bin contents of the total histogram). I also implemented the error computation (uncorelated accross bins).

Quentin
Since it is probably common that the user wants the efficiency and error together, I think it makes more sense to return the errors from the ``overall_efficiency`` method. Then this wouldn't require the bin merging and creation of the temporary Efficiency object twice.

Just change the return statement in the ``overall_efficiency`` method to:

```
return tot_eff.GetEfficiency(1), tot_eff.GetEfficiencyErrorLow(1), tot_eff.GetEfficiencyErrorUp(1)
```

and remove the ``overall_errors`` method.