Deprecate 'get_area_extent_for_subset' and fix incorrect return value

[djhoese]

Sphinx was complaining about bad type definitions in the docstring of 'get_area_extent_for_subset'. I decided to move the types to normal type annotations since we use that elsewhere. This revealed that this method hasn't returned a tuple of scalars since 2018. This method was added in 2018 in #44 and then later we changed how the get_proj_* methods work and dropped this scalar use case. Since no tests were ever added for this method the breakage was never caught. In this PR I fix the type annotations and deprecate the method in favor of the slicing logic available for AreaDefinition to create a subset area and then get the area extent. Oh yeah, Claude pointed out that this method was on the BaseDefinition but was using methods that only existed on the AreaDefinition.

I also added type annotations to a helper function when I was confused about how I wanted to replace the internals of this method.

Bottom line: This method has been broken for years, it is being removed.

CC @meteoswiss-mdr who originally added in in #44 (Ulrich)

• Closes #xxxx
• Tests added
• Tests passed
• Fully documented

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.71%. Comparing base (8aca19c) to head (f4a479f).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
pyresample/geometry.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #749      +/-   ##
==========================================
+ Coverage   93.68%   93.71%   +0.03%     
==========================================
  Files          89       89              
  Lines       13721    13716       -5     
==========================================
  Hits        12854    12854              
+ Misses        867      862       -5     

Flag	Coverage Δ
unittests	93.71% <60.00%> (+0.03%)	⬆️

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[pnuu]

LGTM. One comment inline.