added random seed option for double well

[brookehus]

I decided it was easier to add a parameter to init but let me know if you prefer something different. What kind of tests do you want for this?

• Make sure to include one or more tests for your change
• Add yourself to AUTHORS
• Add a new entry to the doc/source/CHANGELOG (choose any open position to avoid merge conflicts with other PRs).
  Decide whether your change is a fix or a new feature.

[brookehus]

cc @thempel

[codecov]

Codecov Report

Merging #1388 into devel will decrease coverage by 0.01%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##            devel    #1388      +/-   ##
==========================================
- Coverage   91.55%   91.54%   -0.02%     
==========================================
  Files         225      225              
  Lines       25450    25454       +4     
==========================================
  Hits        23302    23302              
- Misses       2148     2152       +4

Impacted Files	Coverage Δ
pyemma/datasets/double_well_thermo.py	94.25% <88.88%> (-0.93%)	⬇️
pyemma/util/statistics.py	59.4% <0%> (-1%)	⬇️
pyemma/coordinates/data/util/traj_info_backends.py	90.61% <0%> (-0.94%)	⬇️

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[marscher]

Should I wait for a test or not? This seems like a minor change, which is not really exposed to user api.

[brookehus]

If there should be a test let me know what I should test. I don't mind either way.

[marscher]

LGTM