Refactored fields to support any # of fields.

The field class is now a wrapper that contains a vector of any number of
fields at once. So now you can choke on smoke, burn in fire, and melt in
acid all at the same time. Fields now propagate additively... so if
something is spewing smoke instead of overwriting the old smoke, it adds
to its density.
Fixed some weird spreading behavior with smoke and fire.

Smoke now lasts longer and generates faster indoors, but small fires are
much less likely to destroy a building (density 1 fires). Fire will last
a more reasonable length of time.
Comments and monster field changes

If a field spawns ontop of you you will now be hit by it immediately.
Also commented things.
Fixes NPC Morale Penalty
Merge multi-field per tile support.

Conflicts:
	field.cpp
	game.cpp
	inventory.cpp
	mapdata.h
	npc.cpp