Update NEON surface data PFT mixture to match NEON observations

It would be great if we could use the NEON observational data (e.g. inventory data) to determine the percentage of each PFT in the surface data files we use to drive NEON cases. This is especially important for FATES work in some of the reduced complexity modes (e.g. fixed biogeography).

NEON has data from ~20 inventory plots surrounding each tower so my idea would be to use this data to determine the mixture of PFTs at the sites. We can add this functionality to `modify_singlept_site_neon`.

Similarly, we should also think about creating inventory initialization files for FATES-NEON runs, using similar methods.