Address diffs v2.12.1 to v3 (#907)

**Bug Fixes:**  
- Fixed incorrect logic in `cf_units.Unit` that replaced `genutil.udunits`, which previously caused incorrect results.  

**Performance Improvements:**  
- Improved subsetting logic by performing a time slice before loading time series datasets into memory, enhancing performance by reducing the size of the datasets
- Optimized variable derivation by subsetting and loading datasets into memory with `.load(scheduler="sync")` first, as the `convert_units()` function requires in-memory access for `cf_units.Unit` operations -- affected `MERRA2` datasets
- Addressed an issue with single-point time dimensions in climatology datasets (e.g., OMI-MLS). xCDAT now squeezes the time dimension to remove time coordinates and bounds before loading the dataset, avoiding errors like "Non-integer years and months are ambiguous and not currently supported." -- affected `OMI-MLS` datasets
- Update `_get_slice_from_time_bounds()` to load time bounds into memory if they already exist using `.load(scheduler="sync")` to avoid hanging -- affected `streamflow` datasets
- Add missed unit conversion for `H2OLNZ` via #874 to address large differences
  - Related code: https://github.com/E3SM-Project/e3sm_diags/blame/ca41b0e5d913610c88410928951f1ed11c75663f/e3sm_diags/driver/zonal_mean_2d_driver.py#L166-L171

**Testing:**
- Performed regression testing with v2 and v3 data. Results check-out, but a closer look will be done after v3.0.0rc1 release.

---------

Co-authored-by: tomvothecoder <[email protected]>

Author: chengzhuzhang

auxiliary_tools/cdat_regression_testing/843-migration-phase3/run-script-model-vs-obs/run_script.py

@@ -12,7 +12,7 @@
     Or: source /lcrc/soft/climate/e3sm-unified/load_latest_e3sm_unified_anvil.sh
 
 NERSC perlmutter cpu:
-    salloc --nodes 1 --qos interactive --time 01:00:00 --constraint cpu --account=e3sm
+    salloc --nodes 1 --qos interactive --time 2:00:00 --constraint cpu --account=e3sm
     source /global/common/software/e3sm/anaconda_envs/load_latest_e3sm_unified_pm-cpu.sh
 """
 # flake8: noqa E501
@@ -67,7 +67,9 @@ def run_all_sets():
         "ANN",
         "JJA",
     ]  # Default setting: seasons = ["ANN", "DJF", "MAM", "JJA", "SON"]
-    param.results_dir = "/global/cfs/cdirs/e3sm/www/cdat-migration-fy24/843-migration-phase3-perf-benchmark"
+    param.results_dir = (
+        "/global/cfs/cdirs/e3sm/www/cdat-migration-fy24/25-01-10-907-v2-data-2"
+    )
     param.multiprocessing = True
     param.num_workers = 24