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.

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• 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 #414