Assign FATES columns and patches to their own natural vegetation landunit

This came up as @billsacks idea in #1046. This seems to me to be worthwhile. Currently FATES are on the natural veg landunit, but they are completely different than the normal natural veg. landunit. So it would make sense to have them on a different land unit. Both for documentation purposes, and for the code to have the ability to handle both types at the same time which might be a useful feature. This would make it easier to just run over normal natural veg landunits, or over FATES landunits.