Make reduce output a catalog

[hombit]

Feature request

It would be really nice if Catalog.reduce() would return a catalog object, so it can be spatially-joined or dumped to the disk. Also it would be great, if we can make it assign new columns back to the original catalog (or its copy), because it is a very popular use-case.

Before submitting Please check the following:

• [x] I have described the purpose of the suggested change, specifying what I need the enhancement to accomplish, i.e. what problem it solves.
• [ ] I have included any relevant links, screenshots, environment information, and data relevant to implementing the requested feature, as well as pseudocode for how I want to access the new functionality.
• [ ] If I have ideas for how the new feature could be implemented, I have provided explanations and/or pseudocode and/or task lists for the steps.

[dougbrn]

@smcguire-cmu https://github.com/astronomy-commons/lsdb/pull/414