Fix Scene.crop using PROJ definition to create target area definition

[djhoese]

This issue is described in #1124. While cropping for ll_bbox is not implemented for anything other than the geos projection, it should be fine to pass xy_bbox. @meresmata was still getting a NotImplementedError even after passing xy_bbox. After some testing by @mraspaud, we discovered that the destination area was using src_area.proj_dict to create the new AreaDefinition. For the case in #1124 which uses an AreaDefinition based on an EPSG code, this results in pyproj expanding the EPSG code definition to a PROJ.4 dictionary. This new dictionary is not the exact same (string-wise) as the original EPSG based definition (EPSG:32630).

This PR fixes this by using the pyproj CRS object if it exists.

• Closes Crop scene of visual spectrum of the sentinel 2 satellite #1124
• Tests added
• Tests passed
• Passes flake8 satpy

[ghost]

DeepCode's analysis on #c2e507 found:

1 minor issue. ✔️ 0 issues were fixed.

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[coveralls]

Coverage decreased (-2.0e-06%) to 89.476% when pulling c2e5075 on djhoese:bugfix-crop-epsg into 8853aac on pytroll:master.

[codecov]

Codecov Report

Merging #1125 into master will not change coverage by %.
The diff coverage is 89.47%.

@@           Coverage Diff           @@
##           master    #1125   +/-   ##
=======================================
  Coverage   89.47%   89.47%           
=======================================
  Files         198      198           
  Lines       29210    29229   +19     
=======================================
+ Hits        26136    26153   +17     
- Misses       3074     3076    +2     

Impacted Files	Coverage Δ
satpy/tests/test_scene.py	99.71% <88.88%> (-0.14%)	⬇️
satpy/scene.py	90.19% <100.00%> (+0.01%)	⬆️

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[mraspaud]

LGTM