feat: Allows a Pastel model to be created from a dict of feature names to weights

[dcorney]

This means that a model can be loaded externally from a file and then passed in to create a Pastel model

Part of allowing org-specific models

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Pull request checklist

• I have linked my PR to an issue
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• I have used Claude to /review my PR, and reviewed any LLM generated code myself
• Where appropriate, I have added or updated tests
• Where appropriate, I have updated documentation to reflect my changes

[github-actions]

❗ The branch name allow-org-specific-pastel-models-ai-2050_library does not follow the pattern [issue-number]-description-here.

In order to correctly link this PR to an issue, the branch name should be prefixed with the relevant issue number.

Please recreate this PR using required branch name structure (e.g. 42-allow-org-specific-pastel-models-ai-2050_library).

[dearden]

This looks like a good change! Don't see any issues