Add get_available_criteria

[alex98247]

Summary

Add get_available_criteria for returning available goodness-of-fit statistic classes and move short-code lookup to get_available_criteria_codes.

Quick changelog

• Renamed the previous get_available_criteria behavior to get_available_criteria_codes.
• Added a new get_available_criteria method returning concrete AbstractGoodnessOfFitStatistic subclasses.
• Added explicit loading of goodness-of-fit statistic modules before subclass discovery.
• Updated statistic utility tests for both class lookup and short-code lookup.

What's new?

This PR separates two related utility behaviors:

• get_available_criteria(distribution) now returns the available non-abstract goodness-of-fit statistic classes for the requested distribution.
• get_available_criteria_codes(distribution) preserves the previous behavior and returns the list of statistic short codes.

The implementation also explicitly imports pysatl_criterion.statistics.goodness_of_fit before walking AbstractGoodnessOfFitStatistic.__subclasses__(), so criteria discovery no longer depends on prior import order.

Validation added in tests/utils/test_statistic.py checks that:

• short-code results remain unchanged for supported distributions;
• returned criteria are subclasses of AbstractGoodnessOfFitStatistic;
• returned criteria are concrete, non-abstract classes;
• returned criteria match the requested distribution.