lesson on functions (ongoing)

Dear friends, please find in this commit the lesson on functions. It is under development. So far, circa 70% of the lesson is already done in the .rmd. The autograder is not yet done because I'm having some trouble with it. I've left some comments in the .rmd asking for some support (just search for "need help here") hahahaha. Please, let me know if this is turning out as you want! @laure.vancau @kendavidn @eduardocorrearaujo