Consider adding a warning if the base world does not have the physics plugin loaded


## Desired behavior
Currently if a world level plugin is added, the default plugins do not get loaded. This is fine for advanced users, however for novices this is really confusing. 

## Alternatives considered
- Just print a `gzwarn` or `gzerr` in the even the physics plugin is not running.
- Print a warning if no system is running in `Update` phase.
