Support Euler angles rather than only RPY in RobotModel

[cmower]

We support RPY Euler angles, e.g.:

optas/optas/models.py

Lines 796 to 798 in 9680f57

	def get_global_link_rpy(self, link, q):
	"""Get the Roll-Pitch-Yaw angles in the global frame."""
	return Quaternion.fromvec(self.get_global_link_quaternion(link, q)).getrpy()

However, it would be good to support general Euler angles with a similar interface to as_euler from Scipy.

[cmower]

Closing this, since #113 is the plan for future developments.