Allow user-made multivariable commodity stream definitions

Allow user-made multivariable commodity stream definitions

Currently, when using multivariable commodity streams, users must add them to commodity_stream_definitions.py which involves directly editing the H2I source code.
This is fine for the time being as this is a relatively new feature, but we should probably allow for a slick way for users to define their own multivariable streams.

Proposed solution

We could have the multivariable streams defined in the plant_config? There's not a spot that jumps out as great to me. Maybe it's a Python file (like we do for custom models) that's just a dict with the multivariable commodities? I might like that more so there's less yaml->python translation.

Alternatives considered

The obvious alternative is to not allowed custom multivariable commodity streams.
I don't think we strictly need to allow user-defined ones, especially because the entire capability can be defined outside of the multivariable stream handling (just use one-by-one streams instead).

Lastly, I don't think it's a huge burden for users to add their own multivariable stream definitions to the existing python file and submit a PR. We're an open-source tool, after all!

Additional context

Start with commodity_stream_definitions.py and the plant_config and custom-model configuration patterns mentioned in the issue. Compare the proposed configuration approaches and define a supported way for users to register multivariable streams without editing H2I source. Done means a user can supply a definition and use the multivariable stream successfully.