Introducing additional hyperparameter tuning metrics

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).