get_rotation_period of Linkage does not work for negative angle cranks

[HugoFara]

Using an crank with a negative rotation would prevent this method to work. freq = round(tau / j.angle) should be replaced by freq = round(tau / abs(j.angle)).

For now users can defined and angle above pi to solve this issue (instead of using an angle of -0.1, use 2*pi-0.1).