Adds mechanical pump water wells.

Constructable (takes a long time and a lot of materials) and appears at
farms.
Pump unit difficulty tweak.

Assembling the pump unit shouldn't be that hard. Making a stable well
cavity is the hard part.
Merge branch 'wells' of https://github.com/Rivet-the-Zombie/Cataclysm-DDA into Rivet-the-Zombie-wells

Conflicts:
	data/json/recipes/recipes.json
	data/json/terrain.json

Hopefuly unmushed the rock walls and the well.