Add option to display a progress bar when loading a BalancingLearner

[basnijholt]

This adds an option with_progress_bar to the BalancingLearner.load method.

Output example

I am currently trying to load 370k learners which takes more than 30 min on a slow NFS. It would be really useful to display the progress.

[akhmerov]

I think it's a bad design to expose this to a library user: a good interface should be simple, and this is a customizable non-essential functionality that also adds an extra dependency.

[jbweston]

I agree with Anton; I think that this is a functionality that you could implement independently from the library (given that load is very simple)

[basnijholt]

Thanks for your feedback, I've added it to adaptive-scheduler where it makes more sense because you are dealing with many learners there.