Fix Scene.crop with RGBs and multidimensional data

[djhoese]

Geo2Grid user noticed an odd behavior when making AHI RGBs and cropping to a lat/lon bbox. After investigating I discovered that Scene.crop is broken for any DataArrays with multiple dimensions where y and x are not the first dimensions.

• Tests added
• Tests passed
• Passes git diff origin/master -- "*py" | flake8 --diff

[coveralls]

Coverage increased (+0.05%) to 78.662% when pulling 8e471fe on djhoese:bugfix-crop-rgbs into 757a052 on pytroll:master.

[codecov]

Codecov Report

Merging #664 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #664      +/-   ##
==========================================
+ Coverage   78.63%   78.66%   +0.02%     
==========================================
  Files         138      138              
  Lines       20332    20353      +21     
==========================================
+ Hits        15989    16011      +22     
+ Misses       4343     4342       -1

Impacted Files	Coverage Δ
satpy/scene.py	89.13% <100%> (+0.18%)	⬆️
satpy/tests/test_scene.py	99.47% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 757a052...8e471fe. Read the comment docs.