Added gross_output to Country and GlobalDemoModel

Simply  calculated as Country.x.sum()
gross_output now has with_RoW option
set_final_demand has optional 'recalculate' param
Country series from_sector renamed

The Country.series, f, e, x, and i all had 'from_sector' as the index
name. This doesn't make any sense in these contexts and has been renamed
to 'sector'
model.final_demand column renamed

from country_name to country_iso3
added tolerance param to recalculate_world

recalculate_world now has an optional parameter to specify the stop
value for the iteration. The default is __DEFICIT_TOLERANCE__ which is
currently set to 1
Added tolerance to recalculate_economy
Renamed B to Z and i to m
added calculate_deltas to Country

The globaldemomodel object can't return the deltas yet though.
added calculate_deltas to GlobalDemoModel
Deltas can now be calculated

The model now holds its previous state following a change and,
optionally, deltas can be calculated.
change Z to a function
Added adjacency_matrix()

new method to create an adjacency matrix for all flows, both
sector-sector and country/sector-country/sector using y = axdp