Interpret ExternalImpulse/ExternalForce torque in correct units

[GinjaNinja32]

force (kg m s⁻²) gets scaled down when passing to rapier
torque (kg m² s⁻²) should also be scaled down by a factor of scale²

without this change, there is AFAICT no combination of units that can be passed to ExternalImpulse::at_point/ExternalForce::at_point to give both correctly-acting force and torque with a non-1.0 scale; with this change, passing all values in scale units results in an impulse/force also in scale units, which is then correctly scaled before being passed to rapier.

[GinjaNinja32]

no combination of units that can be passed [...] to give both correctly-acting force and torque with a non-1.0 scale

i was slightly wrong here, but it's silly; if you pass force/impulse in scale units and point/center_of_mass in inverse scale units, you get numbers out such that the linear force/impulse is in scale units and the torque is in SI. it'd be super weird to do the conversion, but it is technically possible.

[sebcrozet]

This should no longer be relevant since we don’t scale internal rapier structures based on the physics-scale/length unit any more. So the user is expected to provide force/torque taking into account their custom units.