MatrixJointModelTransform: missing license + javadoc

[capdevon]

This PR updates the MatrixJointModelTransform.java file with the following changes:

• Adds a copyright header at the top of the file.
• Improves the class-level JavaDoc to better explain the purpose and usage of MatrixJointModelTransform, including details about non-uniform scaling and matrix accumulation.
• Updates field-level comments for clarity.
• Refactors code for better readability by introducing local variables for parent transforms in relevant methods.

These changes enhance code documentation and maintainability without altering functionality.