Rendering the stat functions on MPAS format datasets

[dkang2]

Is your feature request related to a problem? Please describe.

The stats functions in VERDI, such as sum, mean, work well with CMAQ/WRF format datasets, but when displaying MPAS format datasets, these functions don't seem to have rendered.

Describe the solution you'd like
I would be nice to have these stats functions available to MPAS format datasets.

verdi -f /work/MOD3DEV/dkj/Verdi/example_files/WWLLN_mpas120_60min_20230801.nc