Symbolic Derivatives with SymPy do not work

[traikodinev]

As of the latest version, the example is broken.

https://github.com/JuliaRobotics/RigidBodyDynamics.jl/blob/master/examples/6.%20Symbolics%20using%20SymPy/6.%20Symbolics%20using%20SymPy.jl

Using version 2.2.0 works.

The issue is that Quaternion does not support type Sym. I can update later with more info.