Scent refactor part1

I'm interested in changing how the scentmap works, so I decided to learn how it works now. A lot of these changes are for readability, except for flag lookup separation: https://github.com/wormingdead/Cataclysm-DDA/commit/9314db29e5845dcb45bca258ec53b5850fec8946 and the temp_scent refactor 1 through 7.

As far as I can tell for the temp_scent refactor, there shouldn't be more precision loss than before due to integer division. I split up the simplifications to make it easier to follow, but let me know if I need to be even more explicit. (Or if I made a mistake!)

This is part 1 because I also fixed the sizes of the intermediate matrices, but that ended up being a big complicated commit. See last four commits in this branch: https://github.com/wormingdead/Cataclysm-DDA/compare/scent-refactor-part2. I will create a part2 PR if this one goes well.
Not sure I agree with either -- until you pointed out that 90/5 was a minimal loss factor I didn't see that, and squares_used seems equally opaque in either variant to me.

But then, my not-math background is to blame for most of my confusion, I'm sure.

In either case, I'd need to spend several minutes thinking through all the calculations to figure out what was going on. Also, can't we factor out `90/5` and use `18` instead?