Fix to return proper results when variables with 0 value are passed into function

[fershad]

Fix for #173

Fixes the situations below:

• Currently, when grid intensity for any system segment is set to 0 the calculation will default to the Global Average Grid Intensity. This is not right, as the user has passed in a value for a reason and this should be respected in the calculations.
• Currently, when the data reload percentage is set to 0 the default value of 0.02 is used instead.
• Currently, when the first or returning visitor percentage is set to 0 the default values (0.75 and 0.25) are used instead.