Group agg rework

[lintangsutawika]

1. By default, group will not feature the aggregate scores of their subtasks.
2. To show an aggregate, a group_config will need to be defined in the yaml that consists of aggregate_metric (True/False, default False) and weight_by_size (True/False, default False).
3. Use task_id in ConfigurableGroup and ConfigurableTask to be used as identifier in lieu of task name/group name.

[lintangsutawika]

@haileyschoelkopf I've only added the group_config to MMLU tasks and flan_held_in. Let me know what other benchmarks that would need the aggregation to be added back in.

[haileyschoelkopf]

Test failures are due to the changed printing tests--they seem to be comparing against the old main files in tests/testdata. Merging!