Sql optionals

[bojanserafimov]

Implemented:

• tag in optional scope
• filter in optional scope
• mandatory traverse in optional scope

todo:

• Remove the hack (abusing python features)
• Rewrite ([FeedingEvent_3].uuid IS NOT NULL) = 0 as [FeedingEvent_3].uuid IS NULL

[bojanserafimov]

This is a working solution with 2 hacks. Before I address them, I want to first see where the @recurse logic fits in, to avoid going back and forth undoing my work.

If you think these hacks should be addressed before this PR merges, I'll come back to it after I know where the rest of the pieces in the SQL backend fit in.

[obi1kenobi]

Assuming #531 should merge first, and then we work on removing the hacks here?

[obi1kenobi]

Takeaways from in-person conversation:

• Let's ignore the lowering of ([FeedingEvent_1].uuid IS NOT NULL) = 0 into [FeedingEvent_1].uuid IS NULL for now, and just open an "enhancement" issue describing what needs to be done.
• Let's remove the came_from hack and use a dict mapping aliases to came_from values. We might need to pass additional info to to_sql() methods as a result.

After those two steps, this is ready to merge.

[obi1kenobi]

Looks good! A few suggestions to improve the maintainability of the code.