Improve step 3 of the notebook

[babakkhavari]

Even after removing a print statement it prints out a zero, I am not sure why:

[babakkhavari]

Similar to the rural_hh_size and urban_hh_size, make sure that we have cost_of_carbon_emissions rather than the current cost of carbon emissions in the files

[babakkhavari]

These error should be fixed now as well

[camiloramirezgo]

Things done:

• Added "." as the default value of output_directory in the main model class, so we don't have to specify it in the begining of step 3.