Ice content (CR)

This PR contains new content geared towards cold weather. I worked on it a few months ago but lost steam; for now I am at it again. 

Cygwin does not allow me to launch the game yet (outstanding locale bug or something), so I can't playtest right this moment. Last time I played with this content, it seemed okay. 

I had to copy paste by hand most of the code because of the numerous refactorings, so if I seem to have left something out, let me know!

Changes and additions

* A new function calculates radiant temperature, allowing monsters to have an "aura" of hot or cold. Also affects how fire factors into body temperature.

* Monster speed is affected by temperature. Species influence how affected monsters are.

* New tiles added : frost, mist, ice, snow. Ice increases movement cost, has a chance of causing the player to slip. Lowers dodge and melee skills. Ice mist cools the player.

* Gases spread less randomly: gases cannot move to tiles of greater density; this causes them to spread out as evenly as possible, occupying more space.

* New weapons and effects : ice grenade, ice Molotov, ice rocket launcher, some freeze rays. Ice <weapons> have "ice" explosions that leave behind frost and ice. Freeze <weapons> have a chance of encasing the target in a block of ice. 

* New zombies and effects : a normal ice zombie that has a chilling strike which reduces body temperature, and then a spitter equivalent zombie, but ice instead.