Add support for automatically populating a model's linear velocity if a LinearVelocityCmd is applied to it

[adlarkin]

Desired behavior

When a user applies a LinearVelocityCmd component to a model, the model's LinearVelocity component should be populated (if the LinearVelocity component exists). Currently, we have this behavior implemented for links, but not models. Links also have their WorldPose, AngularVelocity, LinearAcceleration and AngularAcceleration components auto-populated if they exist, so perhaps we could also populate these components for models.

Alternatives considered

N/A (new feature) - a temporary workaround for applying/reading linear velocities of models can be doing this on the model's canonical link.

Implementation suggestion

See #992 (comment)

Additional context

This missing functionality was discovered while addressing #966.

The code for populating pose/velocity/acceleration for links can be found here. It also looks like we also do something similar for sensors/collisions (the relevant code for sensors/collisions can be found here).