Preserve image metadata in xarray <-> Dataset conversions

[gselzer]

Closes #200, #198

But we still need to figure out what "preserving image metadata" means in many cases. With the name, it's obvious, as both data structures have that as state. With other metadata like ImageMetadata, it's less obvious.

But discussion should go in #200

[codecov-commenter]

Codecov Report

Base: 22.23% // Head: 22.23% // No change to project coverage 👍

Coverage data is based on head (ce9d3a5) compared to base (0d9df8e).
Patch has no changes to coverable lines.

❗ Current head ce9d3a5 differs from pull request most recent head 2ea4076. Consider uploading reports for the commit 2ea4076 to get more accurate results

Additional details and impacted files

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##           master     #233   +/-   ##
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  Coverage   22.23%   22.23%           
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  Files          17       17           
  Lines        1781     1781           
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  Hits          396      396           
  Misses       1385     1385           

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