Add support for diffuse IAM in the Array and PVSystem classes - #2845

• Closes Support diffuse component-specific IAM in Array.get_iam() #2812
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• Tests added
• Updates entries in docs/sphinx/source/reference for API changes.
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This PR adds support for diffuse component-specific IAM in the Array and PVSystem classes, laying the groundwork for its later inclusion in the ModelChain class.