874 add leg to anomalous ranges

[gabrielwol]

What this pull request accomplishes:

• add leg to miovision_api.anomalous_range table
• add leg to miovision_api.identify_zero_volumes function (identify legs with zero volume)
• uses grouping sets to simplify the id of different combinations of outages.
• implements bug fixes in join conditions previously under #861.

Issue(s) this solves:

• #874

What, in particular, needs to reviewed:

• [ ] implements bug fixes in join conditions previously under #861: we need to resolve the question of if we want auto-flagged gaps to overlap with any type of existing gap. I believe they should not which is what I've implemented here.
• [ ] need to decide if adding legs here should change the volumes_daily aggregation/filtering process. I think it's fine that an outage on one leg stops the data from appearing in the filtered view - otherwise we would need to add leg as a column to that table.

What needs to be done by a sysadmin after this PR is merged

• [ ] add column to anomalous_ranges
• [ ] update function
• [ ] update existing anomalous_ranges that mention 'leg' / 'approach' to use new leg column.