health_groups: list of all available health groups. Note: health_groups[i] is the spot / perp product pair of health group i where health_groups[i][0] is the spot product_id and health_groups[i][1] is the perp product_id. Additionally, it is possible for a health group to only have either a spot or perp product, in which case, the product that doesn’t exist is set to 0.