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.
The `_here` wasn't bugging me, probably because I've been working with a lot of stupidly-verbose variable names for the past year and a half. I've just sort of gotten used to it, I guess.

The pluralizing is more a stylistic choice, but I think it's caused by an underlying difference in thought patterns: I think of it as asking "if the square at (x,y) *reduces scent*", whereas you're thinking more like "if among the squares that *reduce scent* there's one at (x,y)".

As for `block_scent`, I think I rather like that better, even if all the terrains that block scent are walls. Just make it more explicit what the effect on scent is.