Critical fix for sorting by mean. v0.0.52.

* Calculation of target for mean no longer uses 50th percentile which
  was clearly copy-pasta from the original handling of the median.
* Crude method added to calculate the percentile of the mean for use
  with windowing the percentile_distance parameter.
* Creation of CandidateList no longer sub-sorts (and I'm also unsure why it
  did in the first place).
* Regression test added for sorting by mean.
* CandidateList initialised with and stores value of 'target'.