Add a model object to trace function outputs

[fershad]

Is your feature request related to a problem? Please describe.
This is related to the scenario outlined by @benjaminrancourt in #195 (comment)

Describe the solution you'd like
The proposed solution would add a model object to the output of the perByteTrace and perVisitTrace functions. This would contain information about the model used for the carbon estimates calculation, as well as information about any constants that are in the model. This additional context would be useful for people trying to reproduce the results of a given estimate - Hey, if you want to reproduce these results, here are all the assumptions that has been made.

[fershad]

The expected output would look something like this:

{
        "co2": {
          ...
        },
        "green": true,
        "variables": {
          ...
        },
       "model": {
           "name": "Sustainable Web Design Model",
           "version": 3,
           "shorthand": "swdmv3",
           "constants": {
               ... // a list of constants & their values here
            }
       }
}