[FEATURE REQUEST] Add capability to define state vector elements as CH4 tracers

[msulprizio]

Initial testing indicates running the CH4 simulation with multiple CH4 tracers has a drastic improvement in performance over running multiple CH4 simulations with a single tracer.

We should therefore add an option in the IMI for users to specify if/how to lump Jacobian simulations. The idea would be for users to specify the number of CH4 tracers to include in a simulation, with each tracer representing an element of the state vector. Based on the number of tracers per simulation and the total number of state vector elements, the IMI setup script would then determine how many Jacobian simulations need to be performed.

Notes:

• We may want to finally move the emissions perturbations out of global_ch4_mod.F90 and into HEMCO_Config.rc to simplify things.
• The python scripts would need to be updated to account for the fact that the resulting CH4 concentrations from each Jacobian simulation will be lumped into an unknown number of netCDF files.

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[msulprizio]

This capability is included in PR #193