Helpful error when using an invalid schedule in OutputWriters, Diagnostics, and Callbacks

[navidcy]

A common mistake is to give a Number as a schedule instead of TimeInterval(Number). It'd be much more user-friendly if we have have a function that validates the schedule (and throw an error) because as is now, the first time you encounter the error usually in run! and it's hard to pin-point it.

Another solution is to test-call the schedule when you initialize the simulation (e.g., put it in a try-catch) and throw the error to user?

(discovered with @glwagner)

[glwagner]

I think we do check this now