Refactor fire code

[samsrabin]

The fire code could use some love:

• Minimize duplicated code among modules
• Reduce the number of modules
• Break up large functions
• Get rid of magic numbers
• Minimize number of running means (see this comment)
• Improve commenting
• Improve whitespace

This should wait until this summer's fire updates are complete, and probably also until Fang's multi-day burning code has been implemented.