update write for char and str auxiliary coords

[rhaegar325]

Fix: Properly handle string coordinates with CF-compliant encoding

Summary

This PR fixes the incorrect handling of string/character coordinates during NetCDF file writing. String coordinates (e.g., type in land variables) are now written with proper CF-compliant encoding and remain as coordinates rather than being converted to data variables.

Problem

The current write() method incorrectly handles string coordinates:

1. Coordinates become variables: String coordinates like type are written as data variables instead of remaining in ds.coords
2. Wrong dtype: String dtype changes from |S11 (byte string) to <U11 (Unicode), which is not CMIP6 compliant
3. Missing CF compliance: String coordinates are not encoded using the CF-standard character array format (S1 + strlen dimension)

Impact: Land variables with type coordinate (e.g., baresoilFrac, grassFrac, treeFrac) produce non-compliant CMIP6 output files.

#166 #172 solved

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.52%. Comparing base (1bc3063) to head (48d17bf).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/access_moppy/base.py	90.00%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #174      +/-   ##
==========================================
+ Coverage   60.60%   62.52%   +1.91%     
==========================================
  Files          18       19       +1     
  Lines        2848     2959     +111     
==========================================
+ Hits         1726     1850     +124     
+ Misses       1122     1109      -13     

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[rbeucher]

Tested with baresoilFrac. Works fine. Should we have some simple tests?

[rhaegar325]

Hi, @rbeucher, after some test and fix, this pr is ready to be merged