Forces read only

[Exotik850]

Objective

Fixes #907

Solution

This PR splits the RigidBodyForces trait into ReadRigidbodyForces and WriteRigidBodyForces, and implements ReadRigidBodyForces for the ForcesReadOnlyItem type. This allows for immutable functions to be called on a Forces item immutably retrieved from queries, as shown below.

Testing

Tested by building the example shown in the issue:

fn get_total_inertia(objects: Query<(Forces, &Mass)>) {
    let total = objects
        .iter()
        .map(|(forces, mass)| forces.linear_velocity() * mass.0)
        .reduce(|a, b| a + b);
}

Which now compiles without errors and works as expected.

[Jondolf]

Thanks! I fixed up some of the warnings that were causing CI to fail, so this should be good to merge now :)