Fix gradient resampling to same area not working

[mraspaud]

This implements a noop when source and destination areas are the same for gradient search instead of raising an error.

• [x] Closes #507
• [x] Tests added

[coveralls]

Coverage: 93.799% (+0.004%) from 93.795% when pulling 08b714e5b2e4ae16d0803d22d231a7761ce4028e on mraspaud:fix-gradient-equal-areas into f3e713ebbf95915f6d695cb6a4617d90409dd23b on pytroll:main.

[codecov[bot]]

Codecov Report

Merging #508 (08b714e) into main (0340c4f) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #508   +/-   ##
=======================================
  Coverage   94.27%   94.27%           
=======================================
  Files          78       78           
  Lines       12838    12845    +7     
=======================================
+ Hits        12103    12110    +7     
  Misses        735      735           

Flag	Coverage Δ
unittests	94.27% <100.00%> (+<0.01%)	:arrow_up:

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Impacted Files	Coverage Δ
pyresample/test/test_gradient.py	98.98% <100.00%> (ø)
pyresample/test/test_resamplers/test_resampler.py	100.00% <100.00%> (ø)

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