Add builtin 'config' object and 'features.future_geometry' toggle

[djhoese]

This is the first step of many to allow users a way of transitioning easily between pyresample 1.x and pyresample 2.x interfaces and objects. As a first step this PR adds a new pyresample.config object using donfig and includes a features.future_geometries boolean flag. This flag modifies utility functions like create_area_def to return a future AreaDefinition instead of the old/existing AreaDefinition. Along with some changes to imports users should be able to easily switch usage to the new geometry objects semi-easily (I think).

This also adds a to_legacy() method to the future area so it is easier to create the future area but return the legacy version.

As part of these changes I originally added an import in pyresample/__init__.py so that importing AreaDefinition and SwathDefinition from pyresample would get you the new versions of the classes if this toggle config flag was set. This seemed near impossible to test so I ended up removing it.

TODO:

• Add documentation on configuration

• Modify other utility methods that create AreaDefinitions to use the toggle

• Closes #xxxx

• Tests added

• Tests passed

• Passes git diff origin/main **/*py | flake8 --diff

• Fully documented

[codecov]

Codecov Report

Merging #519 (30305c2) into main (327cec1) will increase coverage by 0.02%.
The diff coverage is 95.69%.

@@            Coverage Diff             @@
##             main     #519      +/-   ##
==========================================
+ Coverage   94.31%   94.33%   +0.02%     
==========================================
  Files          79       82       +3     
  Lines       12936    13001      +65     
==========================================
+ Hits        12201    12265      +64     
- Misses        735      736       +1     

Flag	Coverage Δ
unittests	94.33% <95.69%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pyresample/test/test_gradient.py	99.00% <ø> (+0.01%)	⬆️
pyresample/utils/rasterio.py	62.50% <83.33%> (+8.92%)	⬆️
pyresample/test/test_utils.py	99.37% <97.05%> (-0.63%)	⬇️
pyresample/_config.py	100.00% <100.00%> (ø)
pyresample/area_config.py	90.58% <100.00%> (+0.13%)	⬆️
pyresample/future/geometry/area.py	100.00% <100.00%> (ø)
pyresample/geometry.py	87.50% <100.00%> (+<0.01%)	⬆️
pyresample/test/conftest.py	98.46% <100.00%> (+0.06%)	⬆️
pyresample/test/test_config/__init__.py	100.00% <100.00%> (ø)
pyresample/test/test_config/test_feature_flags.py	100.00% <100.00%> (ø)
... and 3 more

... and 2 files with indirect coverage changes

[coveralls]

coverage: 93.941% (+0.1%) from 93.846% when pulling 30305c2 on djhoese:feature-config-feat-flags into 2b94b70 on pytroll:main.

[mraspaud]

A couple of typos I think, but otherwise LGTM! Thanks for taking the time to cleanup on the way

[djhoese]

Everything should be fixed here, but I'll wait to merge until the various bug fix PRs are merged and released.