Add quaternions.

[aricci10]

A pull request has been done so that the quaternion form can be returned. Could you please accept the pull request? Thanks.

[per1234]

It looks like there are two pull requests that add this functionality:

• Added getYPR, getYPRRadians and getQuaternion. Possible usage for variable integration steps. #18
• Added getQuaternion() #21