Mjcf: Use xyz as a default value for eulerseq compiler option

[JafarAbdi]

This PR initializes mapEulerAngles with an identity matrix by default (matching MuJoCo's default 'xyz' sequence as documented in https://mujoco.readthedocs.io/en/stable/XMLreference.html#compiler-eulerseq).

While testing robots from https://github.com/google-deepmind/mujoco_menagerie, I encountered nan values in some frame transforms. After debugging, I found out that mapEulerAngles was not initialized by default and returned garbage values when no eulerseq was specified.

[hrp2-14]

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[jcarpent]

Thanks a lot @JafarAbdi for this nice contribution. Thanks a lot for adding the test.