Changes to stochastic_benchmark.VirtualBestBaseline.evaluate

[PratikSathe]

In order to obtain confidence intervals for the virtual best baseline, the evaluate function was recently modified.
Some functions that do the same job as some of the functions from stats.py have been defined within the scope of this evaluate function in stochastic_benchmark.py.

It will be more convenient to make this evaluate function directly use functions from stats.py. This way, any changes one might want to make to stats.py will not have to also be made manually in the stochastic_benchmark.py module.