Preserve metadata on write

[jo-mueller]

Built on top of #623 . In a future (and actually already present) ome-zarr-verse, a zarr group can be both a Scene and a plate, i.e., both keys could be present. This PR makes sure that when we are writing to a zarr group in overwrite=False aka append mode, we are not deleting metadata that may have already been there under the ome key.

✅ Added a test to make sure it works

[codecov]

Codecov Report

❌ Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 87.02%. Comparing base (025e83d) to head (db088d9).

Files with missing lines	Patch %	Lines
ome_zarr/classes/scene.py	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #628      +/-   ##
==========================================
+ Coverage   86.91%   87.02%   +0.10%     
==========================================
  Files          17       17              
  Lines        2568     2543      -25     
==========================================
- Hits         2232     2213      -19     
+ Misses        336      330       -6     

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[jo-mueller]

Decision: We will defer this to later until it becomes an actual issue. Right now, nuking the metadata is usually fine.