[FEATURE REQUEST] Evaluate impact on model performance of using multiple methane species vs running multiple methane simulations

[msulprizio]

To construct the Jacobian, we currently run N+1 methane simulations perturbing the emissions for each state vector element. This can result in running a large number of GEOS-Chem simulations. One possible improvement may be to utilize multiple methane tracers in a single GEOS-Chem simulation.

[msulprizio]

This is now discussed in #119 and will be addressed in a forthcoming pull request.