feat: Metadata.from_metadata

[dbirman]

This PR adds a new function Metadata.from_metadata that simplifies the process of generating derived asset metadata. It implements four rules (see the docs page) that control how metadata is inherited.

[tmchartrand]

I'll look through in a bit more detail later, but one high-level thought:
I think we should clarify that we still want to inherit subject metadata if there are additional input assets without subject metadata (ie aggregated reference data, templates or the like) in addition to the single-subject assets (and same for sessions).

Does that make sense @saskiad @dbirman ?

[dbirman]

I'll look through in a bit more detail later, but one high-level thought: I think we should clarify that we still want to inherit subject metadata if there are additional input assets without subject metadata (ie aggregated reference data, templates or the like) in addition to the single-subject assets (and same for sessions).

Does that make sense @saskiad @dbirman ?

For example, if someone uses the CCF to do registration that doesn't mean their derived asset is "multi-subject"? I agree we can add that clarification.

[tmchartrand]

Cool I'm pretty sure the code handles this correctly as-is, but we should update docstrings and docs pages and maybe have explicit tests.