Fix map_reduce calls with utility and add tests

[Xuzzo]

In many examples and tests map_reduce is passed a utility, but this works only when num_jobs <= num_runs. When num_jobs > num_runs, data being a Utility is not supported. Considering we probably want to keep data a simple collection, find a way around calling map_reduce with a utility and test it.

See, e.g. this discussion

Once fixed, unify interface in montecarlo methods (there is a TODO in permutation_montecarlo_shapley)

[mdbenito]

Shouldn't it just be map_reduce(fun, u.data.indices, ...) ? Or is that usage what's broken?

[Xuzzo]

In order not to copy all the data to all the workers, map reduce was splitting the dataset in many different chunks. This is a problem, since each worker was calculating the Shapley values on a different dataset, causing wrong results.
We have now switched to ray which uses Plasma under to hood to share objects with the various workers.

See #9 #22 and #113