Add argument to accessor functions to ignore subsystems

[TorkelE]

E.g. parameters gives a systems all parameters. However, often, it is useful to only get the parameters of a specific system, while ignoring subsystems.

Would it be possible to have a toplevel_only kwarg to these to ignore subsystems (basically replicating get_ps, but something users can use).

I could probably do these implementations in a PR myself, but would want to check with people first if there is any more input.

[ChrisRackauckas]

Yes, it would be good to not expose get_xxxx accessors as much, because they are just getfield calls and so they are a much lower level utility. Giving more options to the standard accessors would be a much better API to document and maintain.