simplifying Julia code for Monte Carlo

[leios]

Hey guys, I screwed up. Sorry. Resubmitting a prematurely merged and then reverted PR... I think I learned my lesson, though!

Copy-paste from #159: a lot of people were mentioning that the julia code for monte carlo was unclear. After looking at it again, I agree. I really wanted to have some parameter radius in the function, just because I wanted to make it clear that the circle could be an arbitrary size. That said, @jiegillet brought it to my attention, that we clearly say we are using a unit circle in the code, and if we read in a radius of greater than the box length, the code is wrong.

For this purpose, I removed the radius parameter from the julia code (except in the in_circle() function, but I can remove it there too). I was just being overly cautious to try to make the text easier to read and understand and in the process actually made it more difficult for new contributors.

Let me know what you think. I should probably just remove the radius from the in_circle() function too, now that I think about it.

We need to put code in that makes sense, so whatever you guys think is best should be what we go with here.

Note: I updated after @Gustorn 's request.