single year example with CO2 objective

[kayaraski]

BUG:
When you run urbs with CO2 objective with single year example, program gives an key error for 'Weight' at modelhelper.py line 86 (stf_dist function).
This is because there is no distinction between intertemopral model and single year model inside co2_rule function. Both try to call stf_dist(stf,m) directly which expects m.global_prob[stf,'Weight'] to exist. That key does not exists for single year example. @sonercandas