Set/assert value function

[janowicz]

Function used to make and track one-off fixes/assertions/look-up-based-corrections to the data.

Examples:

assert(buildings, 'sqft_per_unit','>250')
assert(buildings, 'non_residential_sqft',0,'building_type_id=1')
assert(buildings, 'non_residential_sqft','footprint_area*stories','building_type_id>2')
assert(parcels, 'land_use_type_id',10,'parcel_id==2314')