Distinction between sunlit vs. shaded leaves in Luna (e.g., for ozone stress)

This issue came up in conversations with @danicalombardozzi, @ziu1986, and @billsacks in the context of merging #1370 .

There seems to be a need for properly taking care of the distinction between sunlit vs. shaded leaves when calculating the stress coefficients in LUNA.

See `CTSM_ROOT/src/biogeophys/LunaMod.F90`, and the input variable `o3coefjmax` to the subroutine `Nitrogen_invenstments`.