Refactor scene to privatize some attributes and methods

[mraspaud]

This PR privatise some attributes and methods of the Scene class

• Tests passed
• Passes flake8 satpy

[djhoese]

LGTM

[coveralls]

Coverage increased (+0.005%) to 89.986% when pulling e8f2218 on mraspaud:feature-privatize-scene-items into b86b2f7 on pytroll:master.

[codecov]

Codecov Report

Merging #1300 into master will increase coverage by 0.00%.
The diff coverage is 98.17%.

@@           Coverage Diff           @@
##           master    #1300   +/-   ##
=======================================
  Coverage   89.98%   89.98%           
=======================================
  Files         219      219           
  Lines       32101    32106    +5     
=======================================
+ Hits        28885    28891    +6     
+ Misses       3216     3215    -1     

Impacted Files	Coverage Δ
satpy/scene.py	90.51% <95.04%> (+0.23%)	⬆️
satpy/multiscene.py	90.62% <100.00%> (+0.02%)	⬆️
satpy/tests/reader_tests/test_iasi_l2_so2_bufr.py	97.18% <100.00%> (ø)
satpy/tests/reader_tests/test_modis_l2.py	98.18% <100.00%> (ø)
satpy/tests/test_scene.py	99.72% <100.00%> (ø)

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