global CO2 emissions unnecessary constraint

[oakca]

I am not sure if this is fixed with the new version of the urbs, and/or if following constraints are needed elsewhere; however, I find some interesting thing (I am working atm with an older version of urbs + some constraints are also removed. From my part it is not used elsewhere.):

so with the following constraint urbs calculates the CO2 emission of a powerplant:

c_e_def_process_output(1_Mid_Coal_plant_CO2)_:
+1 e_pro_out(1_Mid_Coal_plant_CO2)
-0.29999999999999999 tau_pro(1_Mid_Coal_plant)
= 0

and then it calculates the global CO2 with following constraint:

c_u_res_global_co2_limit_:
+4380 e_pro_out(1_Mid_Biomass_plant_CO2)
+4380 e_pro_out(1_Mid_Coal_plant_CO2)
...
<= 10000000000

What I am suggesting is that this 2 constraints could be combined into 1. Which would eventually reduce memory usage.

Something like following:

c_u_res_global_co2limit: +4380 eff tau_pro(1_Mid_Biomass_plant) +4380 0.2999 tau_pro(1_Mid_Coal_plant) ... <= 10000000000

Of course if e_pro_out(1_Mid_Coal_plant_CO2) is needed for elsewhere it won't work. But if it is the only place which is used. Then it should be changed.