Support initialization of indexed Set from scalar functions returning dicts

[jsiirola]

Fixes # .

Summary/Motivation:

This is an extended version of #3363: merging this PR will automatically merge & close #3363. See the original PR for an excellent discussion / motivation.

This updated PR contains some minor changes to the original:

• resolve errors when initializing abstract sets from external data
• clarify some changes to the Set documentation
• resolve conflicts with the current state of main

As part of going through this PR, it became apparent that a larger PR should be put together that standardizes how we resolve Initializers in this context (for example, a similar change should probably be added to handling dimen and domain). However, that change started to become more involved, so I am proposing that we fix the current issue here and defer a more general update to the initializers (including finally propagating the use of Initializer to the rest of the Components) to a subsequent PR.

Changes proposed in this PR:

• (see above & Allow initialization of indexed sets via a function returning a dict #3363)

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[mrmundt]

A+ updates to the docs.