Changes and fixes on Adabosot

- This fixes conflicts with master;
- This fixes broken tests;
- This includes some minor changes and cosmits;
- This includes some more tests.

I also think that the examples `plot_adaboost_regression.py` and `plot_adaboost_error.py` would need some more love. In the first one, we don't see that adding more estimators overfit that much. In the second one, the weights are all the same (=1.). I don't really know if that's expected or not?