Fix areacello handler unlimited dimensions error (#319)

Fixes issue where areacello processing fails with "Unlimited dimension(s) {'time'} declared in 'unlimited_dims-kwarg', but not part of current dataset dimensions" error.

The remap() function was unconditionally setting 'time' as an unlimited dimension, but areacello datasets don't have a time dimension since they represent time-invariant ocean cell areas. Now checks if 'time' dimension exists before setting it as unlimited.

Fixes #317

🤖 Generated with [Claude Code](https://claude.ai/code)

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

Author: chengzhuzhang

e3sm_to_cmip/mpas.py

@@ -124,7 +124,9 @@ def remap(ds, pcode, mappingFileName):
     for varName in ds.data_vars:
         ds[varName].attrs.pop("missing_value_mask", None)
 
-    write_netcdf(ds, inFileName, unlimited="time")
+    # Only set time as unlimited if it exists in the dataset dimensions
+    unlimited_dims = "time" if "time" in ds.dims else None
+    write_netcdf(ds, inFileName, unlimited=unlimited_dims)
 
     if pcode == "mpasocean":
         remap_ocean(inFileName, outFileName, mappingFileName)