Add missing stubs to exercises

[dark-wizz]

The problem

Right now for some exercises, a student is required to read the tests to know which functions they should implement. This is annoying, time consuming and might discourage people from doing the exercise entirely. We should provide stubs for all exercises. Stubs should provide the signature of functions and classes, just so the student is immediately ready to start writing the custom logic. This is an example of a stub, from the Raindrops exercise:

class RaindropConverter {

    String convert(int number) {
        throw new UnsupportedOperationException("Delete this statement and write your own implementation.");
    }

}

All stubs must follow this format.

Contributing to this issue

This is a big issue that will require changes to a lot of exercises. The good news is that you don't need to fix this is all exercises to contribute to this issue! PRs fixing this in just one exercise at a time or a couple of exercises at a time are more than welcome.

[andrerfcsantos]

I absolutely agree we should do this!

@dark-wizz I changed the description of the issue just to make the issue and what needs to be done clearer.

[jcovington16]

Can I take this one on?

[andrerfcsantos]

@jcovington16 Go ahead! Are you considering changing this is all exercises or only in a subset of them?

[jcovington16]

Just a subset. After this one I'll be able to knock out some more difficult tasks. Just trying to work myself up to it.

[andrerfcsantos]

Great!

In that case, I won't assign the issue to you, just so other people can also contribute for this in other exercises.

[jcovington16]

Understood. I'll look for a more challenging issue.