fix: safe default zarr write mode

[slevang]

Closes #106. Figured this was clearer than directly passing through a default mode kwarg, since I can't imagine ever wanting to use 'a' or 'r+' here.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d014c87) 94.21% compared to head (0f95d04) 94.19%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #107      +/-   ##
==========================================
- Coverage   94.21%   94.19%   -0.02%     
==========================================
  Files          71       71              
  Lines        5821     5823       +2     
==========================================
+ Hits         5484     5485       +1     
- Misses        337      338       +1     

Files	Coverage Δ
xeofs/models/_base_cross_model.py	92.98% <100.00%> (+0.06%)	⬆️
xeofs/models/_base_model.py	95.37% <100.00%> (+0.04%)	⬆️

... and 1 file with indirect coverage changes

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