Added debug menu command to force a weather change.

I created this while trying to debug #2396 to make it easier to reproduce than "wait for a rainy night". While I ended up getting lost in the curses calls, I think that a debug command to force the weather to a certain type would be helpful for debugging weather-related issues, so I put this up for a merge.

This branch adds a debug menu option to force the weather to a certain type. It takes effect immediately, but doesn't mess with overall long-term weather patterns - it is, after all, for debugging purposes only, and does not attempt to accurately model the effects of a sudden snowfall in the middle of June.