Make use of bounding_box for area freezing when available

[mraspaud]

This PR adds the possibility to use the bounding box attribute of the swathdefinition to freeze a dynamic area definition.

• Tests added
• Tests passed
• Fully documented

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.97%. Comparing base (8cc6ec8) to head (c4ce8ee).
Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #623      +/-   ##
==========================================
- Coverage   93.98%   93.97%   -0.02%     
==========================================
  Files          86       86              
  Lines       13839    13848       +9     
==========================================
+ Hits        13006    13013       +7     
- Misses        833      835       +2     

Flag	Coverage Δ
unittests	93.97% <100.00%> (-0.02%)	⬇️

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[coveralls]

coverage: 93.672% (-0.01%) from 93.682%
when pulling c4ce8ee on mraspaud:feature-bounding-box-2
into b279d35 on pytroll:main.

[djhoese]

It'd be nice to document some of the special xarray .attrs that affect pyresample behavior, but I understand if you don't want it to be in this PR. This type of check could be moved to geoxarray in the future too.