feat: add union operation of theta sketches

[proost]

Adding Union operation of theta sketches.

[freakyzoidberg]

Do you mind naming this ThetaUnion (Union is too generic and this get confusing on the call site when mixing different sketch type)

(I probably missed other case like that, I ll do a pass to qualify those exported types)

Thanks

[freakyzoidberg]

Some context here, the name colision thing is a larger problem - for the Go repo it's going to be easier to address now than later

https://lists.apache.org/thread/4c78gsls7xjrn0b4cosvppywh2clwz6x

[proost]

Some context here, the name colision thing is a larger problem - for the Go repo it's going to be easier to address now than later

Yes, I agree that. But In go, always use package as accessor. And even if packages are conflicted, user can change package name using alias when import.

refs:
https://go.dev/wiki/CodeReviewComments#package-names
https://go.dev/blog/package-names#naming-package-contents
https://go.dev/doc/effective_go#package-names

[proost]

But adding prefix is project or community rule, I will follow it. Do you want still change it?

[freakyzoidberg]

Lets go with Go convention here - thanks