DM-46239: Add pixelization envelopes for Intersection/UnionRegion

[mwittgen]

Checklist

• ran Jenkins
• added a release note for user-visible changes to doc/changes

[erykoff]

First: a typo and a leftover flatten. Second: I'm worried that findPixels is recursive, I thought that there was discussion on the ticket that this is bad? (This may be way out of my wheelhouse though).

[erykoff]

Typo in comment.

[mwittgen]

Allowing to loop over a flattened structures, we wanted to push to another ticket when it's clearer how to define a better API.

[TallJimbo]

Looks good. I wish we had an alternative to a getRegions that deep-copies everything just to let Python iterate over it, but given pybind11's inflexibility re unique_ptr vs. shared_ptr holders for the same type, I imagine we're pretty much out of options.

[TallJimbo]

I think we're fine with letting this continue to be a fully recursive implementation and punt flattening that out to a new ticket.