Allow Cube Query to opt-out of flattening redundant children

[TomTirapani]

See issue: #3843

Seemed like a quick win, that would allow us to clean up some code at a flagship client app. Wasn't sure about the name, its tricky to describe what this does in just a couple of words - any suggestions welcome!

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[lbwexler]

We discussed, and I like this change al ot, as it also helps document a bit of a subtle behavior.

One unfortunate/confusing thing is that it is a lot like an omitFn, and I suppose we might have been able to implement it as an omitFn, although because of its recursive nature of zapping multiple nodes in a chain, I am not sure how straightforward that would be.

Perhaps we should call it omitRedundantNodes or flattenRedundantNodes?

[lbwexler]

I think it really is an omitFn. We could make it the default omit function on cube e.g. Cube.omitFn = omitRedundantNodes but that would be a breaking change for anyone overriding the omitFn, who would now have to also refer to the default implementation.

Maybe best to just call it omitRedundantNodes and have omitFn and omitRedundantNodes just mention each other in their docstrings.