Add LinkItem type; Update model_params field to support new LinkItem type

[aaraney]

fixes #72

ngen.config - [0.2.0]

Additions

• Add LinkItem type to capture the concept of a source and a variable from that source.

Changes

• BREAKING: BMIParams's model_params field no longer supports Mapping[str, str]. This was never supported by NextGen. See ngen-config Update model_param spec #72 for more information. Supported value types are now int, float, sequence[int], sequence[float], LinkItem.
• BMIParams's (and subtypes) model_params field now supports LinkItem as a mapping value type.

Checklist

• PR has an informative and human-readable title
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• Code can be automatically merged (no conflicts)
• Code follows project standards (link if applicable)
• Passes all existing automated tests
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future todos are captured in comments
• Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)
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[aaraney]

@hellkite500, changing the types accepted by the model_params is a breaking change as I removed Mapping[str, str]. That should merit a major version bump, however since we are still <1.0.0 im fine with only bumping the minor version. What are your thoughts?

[aaraney]

@hellkite500, I assumed you would agree with me about the minor version bump, so for getting this merged more quickly, I went ahead and pushed that change. JLMK if you want me to roll it back and do something different. If not, feel free to review and merge. We should be good to go.