Restructure output datasets

Generalize output formats, the metrics are generated for these four aggregation levels

• metrics_by_category: metrics for content gap categories (e.g. female category of the gender gap)
• metrics_by_content_gap: metrics for content gaps (e.g. across all categories)
• metrics_by_category_all_wikis: metrics across all wikis per category
• metrics_by_content_gap_all_wikis: metrics across all wikis per content gap

Additional changes

• improved configuration, including adding a sub_content_gaps arg to select specific gaps to compute
• updated validation notebook to use new output files
• bump version to 0.3.0