math_parser/eoc: migrate arith. weather to math

[andrei8l]

Summary

None

Purpose of change

Port another arithmetic function to math. weather was used recently for #64954 and it's a small enough target.

Describe the solution

Move the code from arithmetic to math.
Use Kelvin for temperature to match internal representation.
Add temperature conversion functions.

Fix the calculations and conditions for fog/mist: ported from EOC to jmath_function. Added reference for the formula we used and re-converted it to Celsius. Fixed the conditions for mist to better match reality.

Describe alternatives you've considered

Continuing to use Fahrenheit: Temperature is internally already Kelvin so it makes more sense to expose it to EOCs in the same unit. Weather aspects use a mixed bag of undocumented metric and non-metric units atm and I'd rather port them all to SI if possible.

Testing

Existing test unit.

Both mist and fog still occur in game. Mist/fog no longer occur by default on a fresh game simply because dew_point_factor hasn't been calculated yet.

Additional context

I promised I'd do this for #64954 but I got sidetracked IRL. Better late than never though.

[andrei8l]

Thanks!