[WIP] add balanced_accuracy_score metric #3506

It refers to the issue https://github.com/scikit-learn/scikit-learn/issues/3506

- It passes all the common tests for metrics and regression metrics, but I still need to implement specific tests.
- It might be a duplicate of the work done by @lazywei at https://github.com/scikit-learn/scikit-learn/pull/3511 , so I will compare the two versions
If this is just getting the number of 1s from a binary array, why not use `sum`?

Alternatively, you can get all these measures in a single `bincount`: `tn, fp, fn, tp = np.bincount(y_true * 2 + y_pred, minlength=4)`