Add 'crop' and 'save_datasets' to MultiScene

[djhoese]

This PR adds the 'crop' and 'save_datasets' method to the MultiScene object. It also cleans up some of the dask distributed usage from a previous PR. This adds the functionality to use dask distributed in save_datasets for the MultiScene as well which can improve processing time.

The biggest possible cause of issues from this PR is the work I had to do is to fix the serialization of multiple readers and other parts of satpy. Dask distributed does not like map_blocks or delayed functions to be internal functions because they can't be serialized. This should be fixed in all of satpy now.

I still need to write tests.

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

[codecov]

Codecov Report

Merging #613 into master will increase coverage by 0.19%.
The diff coverage is 81.28%.

@@            Coverage Diff             @@
##           master     #613      +/-   ##
==========================================
+ Coverage   78.13%   78.33%   +0.19%     
==========================================
  Files         136      136              
  Lines       19615    19714      +99     
==========================================
+ Hits        15327    15442     +115     
+ Misses       4288     4272      -16

Impacted Files	Coverage Δ
satpy/readers/goes_imager_hrit.py	51.65% <0%> (ø)	⬆️
satpy/readers/msi_safe.py	2.52% <0%> (ø)	⬆️
satpy/readers/hrit_jma.py	98.43% <100%> (ø)	⬆️
satpy/tests/test_multiscene.py	97.47% <100%> (+0.98%)	⬆️
satpy/readers/electrol_hrit.py	34.4% <33.33%> (+0.25%)	⬆️
satpy/readers/olci_nc.py	43.42% <33.33%> (+0.57%)	⬆️
satpy/resample.py	88.95% <33.33%> (ø)	⬆️
satpy/readers/slstr_l1b.py	60.99% <40%> (+0.99%)	⬆️
satpy/readers/sar_c_safe.py	20.65% <50%> (+0.54%)	⬆️
satpy/composites/crefl_utils.py	84.52% <50%> (+1.19%)	⬆️
... and 3 more

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[coveralls]

Coverage increased (+0.2%) to 78.33% when pulling 68b8f70 on djhoese:feature-mscn-crop-save-datasets into 71d215c on pytroll:master.