prevent overwriting processed variables during execute_pp_functions

[jtmims]

Description
Variables would overwrite the processing done by the execute functions in the preprocesser for the previous variables. Rewrote the process function to place processed variables into the cat_subset directly. This fixes issues with “pr”.

Associated issue # (replace this phrase and parentheses with the issue number)

How Has This Been Tested?
Please describe the tests that you ran to verify your changes in enough detail that
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such as the Python version, package versions, expected POD wallclock time, and the
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Checklist:

• My branch is up-to-date with the NOAA-GFDL main branch, and all merge conflicts are resolved
• The scripts are written in Python 3.11 or above (preferred; required if funded by a CPO grant), NCL, or R
• All of my scripts are in the diagnostics/[POD short name] subdirectory, and include a main_driver script, template html, and settings.jsonc file
• I have made corresponding changes to the documentation in the POD's doc/ subdirectory
• I have requested that the framework developers add packages required by my POD to the python3, NCL, or R environment yaml file if necessary, and my environment builds with conda_env_setup.sh
• I have added any necessary data to input_data/obs_data/[pod short name] and/or input_data/model/[pod short name]
• My code is portable; it uses MDTF environment variables, and does not contain hard-coded file or directory paths
• I have provided the code to generate digested data files from raw data files
• Each digested data file generated by the script contains numerical data (no figures), and is 3 GB or less in size
• I have included copies of the figures generated by the POD in the pull request
• The repository contains no extra test scripts or data files

[wrongkindofdoctor]

@jtmims Thanks for the fix! I think this workaround will get you through testing, though it may not apply the conversion factor to the data, so the precip plots might be off. I will look into the precipratetofluxconversion execute function to see why the variable is not updating correctly when I am back.