Fix check_group_daterange function in preprocessor.py

[jtmims]

Description
This function was raising an exception due to the fact that an int was being passed to a comparison when it required a datetype. A conversion was added before the comparison was made.

How Has This Been Tested?
Ran on AM5 data on my workstation

Checklist:

• [ x] My branch is up-to-date with the NOAA-GFDL main branch, and all merge conflicts are resolved
• [ x] 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
• [ x] The repository contains no extra test scripts or data files