Fix 'Expression "-self.dq.T * diff(self.D, self.q[i]) / 2" is assigned to nothing' issue in lib\symbolical_dynamics\euler_lagrange.py

[PurplePegasuss]

CodeFactor found an issue: Expression "-self.dq.T * diff(self.D, self.q[i]) / 2" is assigned to nothing

It's currently on:
lib\symbolical_dynamics\euler_lagrange.py:120