Cosim/rigid body solver unittest

[mtnzguillermo]

📝 Description Adding unittests for the Rigid Body Solver

Only a couple of questions:

• Should the big tests run only for a few time steps so the size of the files is not that big?
• Can the file test_rigid_body_solver.py be reduced?

[rubenzorrilla]

Again, why the rigid body solver capability is not being added to the Core? Is there any technical limitation?