Update handling of fates sites area to maintain consistency across fates columns in a given gridcell

[glemieux]

Each fates site is associated with a given naturally vegetated column on a gridcell as has a specific area. Currently we only have one fates site/column per gridcell. In having multiple soil columns per gridcell and passing fates patches across different columns, we need to make sure that the total area of the fates sites is still consistent with the starting area.