Slow down winter changes by default and add the ability to control the changes with 'nexuses'

[oversword]

Add "christmas nexus" and "humbug nexus" that control the winter changes.

The humbug nexus prevents all winter spread in the area around it (radius 25)
The christmas nexus allows the previous winter behaviour in the area around it (radius 25)

Outside of these nexuses, the behaviour has been changed to spread ice and snow from existing ice & snow sources

This means that people's builds will not be ruined against their will, christmas still has a magical effect for those unaware of the nexuses, and those who like the old behaviour have a way to activate it.

This also stops farm water freezing unless you want it too do so.

I suggest we add these nexuses to a christmas shop for a nominal trade, allowing players to trade them back for the same price.

Fixes BlockySurvival/issue-tracker#359

[oversword]

@billy-s @luk3yx I want your review on this, and also your approval before it goes live, I'm not sure my ABMs are the most efficient... if you know a way to make them better please say so.

However, you better have your say before the 21st because that's when the winter behaviour starts, and I will activate this before then if there's no objection.