Several small fixes - Githubissues

This PR fixes several small problems:

It checks whether the CoM velocity in the bodies created with the Newtonian body mixin are indeed zero. Thought this was already the case, and it indeed is. However the location that actually sets the velocity to zero in the body-fixed frame is not within the RigidBody class, but within the System class.
It removes the redundant _parent attributes, which was left in on accident from a previous design (#50).
It fixes the problem with bicycle parameters adjusting the rear frame CoM (#54).
It improves the plotting a bit (#43).

mechmotum / symbrim