This enhancement will bring in more AOD diagnostics against MERRA and MODIS datasets. The observation datasets will reside in the ADF default observation directory at: /glade/campaign/cgd/amp/amwg/ADF_obs/

• MERRA2_192x288_AOD_2001-2020_climo.nc
• MOD08_M3_192x288_AOD_2001-2020_climo.nc

These diagnostics will live in the scripts/plotting/global_latlon_map.py script.

In order for these diagnostics to run, global_latlon_map.py will need to be called in the plotting_scripts section of the run-time config yaml file as well as AODVISdn needs to be included in the diag_var_list.

The new diagnostics will include:

• Panel plots of model vs observation difference and percent difference for seasonal avgs for DJF, MAM, JJA, SON:
  • Test vs Baseline

 - Test vs Obs - will still plot these panel plots, just with the one model case

• Additionally, individual plots for each case and observation will be saved (but not published to the website) for each season and difference and percent difference.

This PR will also add a little more detailing of error handling for history file info in adf_info.py and more flexibility for which season plot is displayed by default in the webpage.

Closes #302

@dwfncar I am tagging you as a reviewer, if you wouldn't mind taking a look at the calculation code at the end of global_latlon_map.py in the function aod_latlon to make sure there are no glaring mistakes. Then we can move this toward the merge, thanks!