Introduce print() fun and comment char earlier

[ErinBecker]

[Submitted by email]

1. Three ways to force R to print the value: using parentheses; typing the object name; using print() function.

The function print() was not introduced in this episode in the current version, possibly because functions haven't been introduced yet; maybe mention this function in the subsection of "Functions and their arguments"? This function is used in the episode "Manipulating, analyzing and exporting data with tidyr" later on.

2. The comment character # should be introduced earlier. In the current setup, comments showed up in the subsection "Objects vs. variables" but no explanation at all there.

3. It would be great to mention that the grammar of the language determines whether an expression is complete or not. e.g. x<- ## incomplete expression

Nothing would happen after entering Enter, because it's waiting for the expression to be completed. This is also a good place to introduce the comment character #.

[Teebusch]

1. Well spotted! I think it would be worth demonstrating the print() function. It would probably fit best in the beginning, with the other 2 ways to print objects. Of course, functions have not been explained there, but it might be ok to say "this is a function. We will learn more about functions soon"
2. True, comments are used shortly before they are explained. I think this is really only an issue if learners are following the lesson by themselves. In a live course, the instructor probably won't copy the comments. Even in a self-learning situation, I think the syntax highlighting makes clear that the comments are comments. Anyways, a solution could be to rewrite this part, so that the comments are included in the regular text instead.
3. This is (to some extent) addressed in interacting with R, albeit the section is not talking about syntax, just saying that "It means that you haven’t finished entering a complete command.". Personally, I think this is clear enough, though.