aggregate: unchunk spatial dimension before apply_ufunc

[FabianHofmann]

Closes #324

Motivation and Context

compat with new xarray version 2023.09.00

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (2555d81) 72.31% compared to head (be66fc0) 72.33%.

❗ Current head be66fc0 differs from pull request most recent head 104d252. Consider uploading reports for the commit 104d252 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #326      +/-   ##
==========================================
+ Coverage   72.31%   72.33%   +0.01%     
==========================================
  Files          19       19              
  Lines        1694     1695       +1     
  Branches      294      294              
==========================================
+ Hits         1225     1226       +1     
  Misses        387      387              
  Partials       82       82              

Files	Coverage Δ
atlite/aggregate.py	75.00% <100.00%> (+2.27%)	⬆️

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[FabianHofmann]

@euronion I took the freedom to uncomment the change logs for the upcoming version