gz sdf --inertial-stats should process world files

[scpeters]

Desired behavior

Currently the gz sdf --inertial-stats command can parse SDFormat files containing a single model and print aggregate inertial statistics (total mass, aggregate center of mass and moment of inertia matrix), but it fails when trying to load an SDFormat world file with Error: Expected a model file but received a world file.

It would be convenient to parse a world file and print the inertial stats for each model contained in the world along with the name of the model.

Alternatives considered

Implementation suggestion

Additional context