aggregate: unchunk spatial dimension before apply_ufunc

[FabianHofmann]

Closes #324

Motivation and Context

compat with new xarray version 2023.09.00

Type of change

• [x] 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[bot]]

Codecov Report

All modified lines are covered by tests :white_check_mark:

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

:exclamation: 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

```diff @@ 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](https://app.codecov.io/gh/PyPSA/atlite/pull/326?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=PyPSA) | Coverage Δ | | |---|---|---| | [atlite/aggregate.py](https://app.codecov.io/gh/PyPSA/atlite/pull/326?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=PyPSA#diff-YXRsaXRlL2FnZ3JlZ2F0ZS5weQ==) | `75.00% <100.00%> (+2.27%)` | :arrow_up: |

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[FabianHofmann]

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