Add helper function area and layout_from_area_density & allow auto chunks

[FabianHofmann]

• add functions area & layout_from_area_density
• allow for auto chunks when loading cutout

Motivation and Context

The area function is helpful for custom workflows, the same with the new layout function.

The auto chunking can help to accelerate the computation of the conversion functions.

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)

Checklist

• I tested my contribution locally and it seems to work fine.
• I locally ran pytest inside the repository and no unexpected problems came up.
• I have adjusted the docstrings in the code appropriately.
• I have documented the effects of my code changes in the documentation doc/.
• I have added newly introduced dependencies to environment.yaml file.
• I have added a note to release notes doc/release_notes.rst.
• I have used pre-commit run --all to lint/format/check my contribution

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (5a1f00f) 72.34% compared to head (d5d2316) 72.49%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #328      +/-   ##
==========================================
+ Coverage   72.34%   72.49%   +0.14%     
==========================================
  Files          19       19              
  Lines        1696     1705       +9     
  Branches      294      296       +2     
==========================================
+ Hits         1227     1236       +9     
  Misses        387      387              
  Partials       82       82              

Files	Coverage Δ
atlite/cutout.py	76.26% <100.00%> (+1.13%)	⬆️

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[euronion]

Minor comments