Improve js code

[loicknuchel]

Here is a proposal to simplify workshop usage : you just have to implement projections and see if it's correct.

[tcoopman]

I'm going to think about this. I'm not sure if we want to go this far. Personally I would like as little code as possible to get people started so that we don't force people into a structure/way of programming. Maybe it would be a good idea to provide some sample solutions in a different branch if we want to do something like this. @michelgrootjans thoughts?

[loicknuchel]

I think having some solutions in a separate folder can be enough, people will not look at them by error ;) (like this: https://github.com/loicknuchel/playing_with_projections/tree/add-js-solutions/js)

But this PR is more about structuring code and giving some feedback if result is OK or not... The only structure here is : a function that take events and returns the expected result. People are free to code how they want inside it.