Aplr

Slightly faster code

Added negative_gini as a validation_tuning_metric.

The following hyperparameter tuning metrics are available:
- "default". This is the default, using the same methodology as when calculating training loss, like in previous APLR versions.
- "mse".
- "mae".
- "rankability".

Deprecated:
- group_size_for_validation_group_mse (constructor parameter).
- get_validation_group_mse (method).

Added a method that returns validation set indexes. This may be useful, for example, if you wish to tune hyperparameters by using custom metrics.