Add sort and remove duplication to statistical_inefficiency

[xiki-tempula]

Fix #118

[codecov]

Codecov Report

Merging #119 (65c4066) into master (0b31fb2) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #119      +/-   ##
==========================================
+ Coverage   97.34%   97.39%   +0.05%     
==========================================
  Files          16       16              
  Lines         942      961      +19     
  Branches      206      211       +5     
==========================================
+ Hits          917      936      +19     
  Misses          5        5              
  Partials       20       20              

Impacted Files	Coverage Δ
src/alchemlyb/preprocessing/subsampling.py	100.00% <100.00%> (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 0b31fb2...65c4066. Read the comment docs.

[orbeckst]

Overall this looks sensible to me. Maybe @dotsdl wants to have a quick look.

I mainly have comments on the tests.

Also add an entry to CHANGELOG, please.

[xiki-tempula]

@orbeckst Thank you for the review. I have updated the PR.

[orbeckst]

@xiki-tempula , this looks good to me. But do you know why the coverage is so low? I see that two if statements are not fully checked (essentially, only the if True code path is tested while the effective else: pass is not). Under which circumstances would these other code paths be triggered?

[xiki-tempula]

@orbeckst Thank you for the review. The two if checks if the series is given and will only modify the series if the series is given.
If the series is not given, the data will be sorted and duplications removed but not decorrelated.

[orbeckst]

Then these should be simple tests: just as before, just don't give the series. It seems a bit over the top but could you please add tests for this case, too? Then this PR will be done.

[xiki-tempula]

@orbeckst Thanks for the suggestions, I have bumped the test.