Add validation for schema YAML files

[yifanmai]

Tasks:

• Add validation function for schema YAML files
• Call validation function in summarize.py i.e. helm-summarize
• Add test that calls the validation function on all schema files included in the HELM package

Things to validate:

• For each group name in subgroups of a run_group, there is a run_group defined with that name.
• For each entry in metric_groups of a run_group, there is a metric_group defined with that name.
• For each entry in metrics of metric_groups, there is either a metric with that name, or it matches the format ${var_name} (where var_name can be any name).
• For each split in metrics of metric_groups, split is either test, valid, __all__, or matches the format ${var_name} (where var_name can be any name).
• (maybe?) Each entry in run_groups is either a parent node (contains category and subgroups) or a child node (contains metric_groups) but not both.