Cannot set initial `ReadMassProperties`

I would like to spawn an entity and query over its mass in the same frame. This does not seem possible today; the only constructor for `ReadMassProperties` is `default()` which sets it to 0.

I understand not wanting this to be mutable in general, but there should be some mechanism to set it initially. The simplest would be to make a `new` function. This could easily be messed up, since the actual properties are derived from the rapier `RigidBody`, but I don't think it would be any worse than today, where it's just 0.0 until the physics systems have run.