More scattering upgrades

[alurban]

This PR implements a few more scattering code upgrades:

• Include more longitudinal-motion OSEMs in the default channel list
• Do not display channels without evidence for scattering on the output page (but still plot them)
• Handle the use case where no evidence of scattering is found in non-empty observing segments
• Provide figure captions for scattering plots
• Include Omicron trigger central frequency and SNR in the scattering summary

An example run analyzing Livingston data from 2019-04-05 is available here (requires LIGO.ORG credentials).

Since these changes depend on the new gwdetchar.scattering module, I'm marking this a work-in-progress until #298 is merged.

This is related to #84 and #296.

cc @duncanmmacleod, @siddharth101, @jrsmith02, @andrew-lundgren

[coveralls]

Coverage increased (+0.1%) to 95.636% when pulling cf014e2 on alurban:scattering-channels into fd8de27 on gwdetchar:master.

[codecov-io]

Codecov Report

Merging #301 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #301      +/-   ##
==========================================
+ Coverage   95.67%   95.67%   +0.01%     
==========================================
  Files          23       23              
  Lines        1569     1571       +2     
==========================================
+ Hits         1501     1503       +2     
  Misses         68       68

Flag	Coverage Δ
#Linux	95.67% <100%> (+0.01%)	⬆️
#python27	95.35% <100%> (+0.01%)	⬆️
#python35	94.84% <100%> (+0.01%)	⬆️
#python36	94.97% <100%> (+0.01%)	⬆️
#python37	94.91% <100%> (+0.01%)	⬆️

Impacted Files	Coverage Δ
gwdetchar/scattering/core.py	100% <ø> (ø)	⬆️
gwdetchar/scattering/__init__.py	100% <100%> (ø)	⬆️

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[alurban]

@duncanmmacleod, now that #298 is merged, this PR is ready for review.

[duncanmmacleod]

LGTM