Check that all exported functions have `@return` and `@examples`

[RobertASmith]

So this is the one thing that I think will take a bit of time.

• Every function with an @export tag needs to have a @return included in the roxygen above. I think most if not all do but we need to check.
• Every function should have an example unless there is a good reason why an example won't work. So far I've been lazy and used lots of dontrun statements so the examples were not executed. More on this here
• I've started this process here