Cleanup: Reduce duplication of fire code

There's substantial duplication among [CNFireLi2014Mod](https://github.com/ESCOMP/CTSM/blob/master/src/biogeochem/CNFireLi2014Mod.F90), [CNFireLi2016Mod](https://github.com/ESCOMP/CTSM/blob/master/src/biogeochem/CNFireLi2016Mod.F90), and [CNFireLi2021Mod](https://github.com/ESCOMP/CTSM/blob/master/src/biogeochem/CNFireLi2021Mod.F90). Once @lifang0209 finishes her current work, let's combine these into one file with conditionals based on the `fire_method` namelist parameter.