PyMJCF enable attachment without wrapping body

[guyazran]

• attach method now has an additional argument exclude_wrapper_body.
  • set to False for previous behavior
  • set to True for new behavior
• internal elements of attachment are positioned and rotated relative to the attachment site.
• aggregated rotation converters to quaternions for relative rotation
• implemented xyaxes_to_quat conversion method
• improved implementation of euler_to_rmat that handles all supported eulerseq values
• implemented generic function wrapper for mju rotation conversions that

resolves #407

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[guyazran]

I realized this implementation is messy and convoluted, and doesn't even support detachment. I now understand the design choice of wrapping the contents within attachment frames, I don't think there is a quick fix that will support my desired functionality. I am closing this PR