KIM Trainer and tests

[ipcamit]

Summary

Contributes the KIM trainer for training openkim models.

• Scipy based optimizer
• Bugfix in Weight class when weight is none but property is not
• Tarball model capabilities
• Removed provider field as each trainer interprets it differently so not needed
• Tests for training new trainer

Checklist

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• [x] Tests have been added for any new functionality or bug fixes.
• [x] All linting and tests pass.

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[mjwen]

Looks alright, nothing needs to be updated for now.