Large IO and tercile class and method restructure

[chiaweh2]

This PR deprecated the old module due to the length and the not user friendly class declaration. In this PR, two main things are addressed

1. The restructure of mom6_process to mom6_io and mom_tercile.
   • The mom6_process is deprecated in the PR
   • local file location is now flexible (user can set the file subdir path under DATAPATH) https://github.com/NOAA-PSL/regional_mom6/issues/22
2. The tercile probability calculation is implemented and included in mom_tercile
   • the gridded method https://github.com/NOAA-PSL/regional_mom6/issues/18
   • the regional averaged method https://github.com/NOAA-PSL/regional_mom6/issues/18

[chiaweh2]

@KevinCounts in case you are using the module. I have made some significant changes again due to the ill design in the beginning. Sorry if this cause any trouble on your end. I am happy to help on the code conversion the new changes is not working on the your version of code implementation.

[KevinCounts]

Great, thanks for the heads up. I'm just working in mom6_indexes right now though, so I should be good.