Unbalanced Steps Sizes Option for Benchmarks Generators

[vlomonaco]

I noticed it is quite tricky at the moment to generate a benchmark with an unbalanced number of examples for each step. It would be nice to have an option in ni_scenario, nc_scenario and similar to set the number of examples for each step.

[lrzpellegrini]

Now we have the data_incremental_benchmark utility that, starting from a benchmark instance, allows for the creation of a new benchmark instance in which experiences are split into mini-experiences of a given fixed size.

I think that making a more abstract version of that helper should work. We can reduce the size of the experiences (thus discarding instances) or we can allow the user to create an unbalanced splitting. @vlomonaco what was your original idea?

[vlomonaco]

Yes I think being able to create an unbalanced split would be ideal maybe just re-distributing examples of the same class within experiences that contain that class? It's difficult to came up with a general solution for this as different benchmarks may need a different "redistribution" policy.