Add group_files and from_files utility functions for creating Scenes from multiple files

[djhoese]

This is partially discussed in #310. Basically when using MultiScene objects or when you want to create a lot of Scenes from an unorganized series of files. I've had to do this "manually" for a lot of the MultiScene examples I've created where doing it requires that I know something about the file pattern. Usually that means knowing when/where the "start_time" field occurs in filenames and grouping files by that time. This PR adds a satpy.readers.group_files utility function to help with this by using a reader's file patterns to figure out the start_time but allow for other fields to be used too.

Additionally, I add a from_files classmethod to MultiScene to use this new group_files method and create a MultiScene. Should I add a classmethod to Scene to do the same (return a list of Scenes)?

I still need to add tests and something in the sphinx docs.

• Closes Create MultiScene from list of files #310
• Tests added
• Tests passed
• Passes git diff origin/master -- "*py" | flake8 --diff
• Fully documented

[coveralls]

Coverage increased (+0.3%) to 77.726% when pulling 87d7ac1 on djhoese:feature-group-files into d011adf on pytroll:master.

[codecov]

Codecov Report

Merging #576 into master will increase coverage by 0.28%.
The diff coverage is 96%.

@@            Coverage Diff             @@
##           master     #576      +/-   ##
==========================================
+ Coverage   77.44%   77.72%   +0.28%     
==========================================
  Files         136      136              
  Lines       19154    19305     +151     
==========================================
+ Hits        14834    15005     +171     
+ Misses       4320     4300      -20

Impacted Files	Coverage Δ
satpy/readers/__init__.py	95.12% <100%> (+0.65%)	⬆️
satpy/tests/test_readers.py	98.41% <100%> (+0.25%)	⬆️
satpy/tests/test_multiscene.py	94.73% <100%> (+0.39%)	⬆️
satpy/multiscene.py	81.13% <84.21%> (+2.01%)	⬆️
satpy/tests/writer_tests/test_geotiff.py	92.64% <0%> (-1.9%)	⬇️
satpy/readers/seviri_l1b_nc.py	17.82% <0%> (-0.63%)	⬇️
satpy/writers/geotiff.py	44.14% <0%> (+0.62%)	⬆️
satpy/tests/reader_tests/test_ahi_hsd.py	96.51% <0%> (+0.79%)	⬆️
satpy/readers/ahi_hsd.py	72.98% <0%> (+11.99%)	⬆️

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[mraspaud]

LGTM