Support multiple columns for `on` in packing functions/methods

[hombit]

Feature request

The motivation: I'm trying to NestedFrame.add_nested on both object ID and passband, and currently I have to go with a work-around with a temporary index.

Before submitting
Please check the following:

• 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]

Potentially want to groupby().add_nested(), on may be confusing in what it should behave as. It could either return a singly-nested with two nested structures, or doubly-nest