Fix Scene save_datasets to only save datasets from the wishlist

[djhoese]

See the associated issue for more details. Basically if you have datasets in your Scene that were not manually added or and aren't in the wishlist (ex. composite prerequisites) then calling save_datasets would save those datasets too which is unexpected. Now a RuntimeError is raised.

• Closes Non-requested datasets are saved when composites fail to generate #332
• Tests added
• Tests passed
• Passes git diff origin/master **/*py | flake8 --diff

[coveralls]

Coverage increased (+0.02%) to 69.153% when pulling 8f31507 on djhoese:bugfix-save-datasets into 9767550 on pytroll:master.

[codecov]

Codecov Report

Merging #342 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #342      +/-   ##
==========================================
+ Coverage   69.12%   69.15%   +0.02%     
==========================================
  Files         123      123              
  Lines       15636    15645       +9     
==========================================
+ Hits        10809    10819      +10     
+ Misses       4827     4826       -1

Impacted Files	Coverage Δ
satpy/scene.py	85.08% <100%> (+0.31%)	⬆️
satpy/tests/test_scene.py	99.31% <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 9767550...8f31507. Read the comment docs.