Integrate Slice Sampling: Generalised Elliptical.

[lorcandelaney]

See #772

[codecov[bot]]

Codecov Report

Merging #900 into master will decrease coverage by 0.39%. The diff coverage is 87.34%.

@@           Coverage Diff            @@
##           master    #900     +/-   ##
========================================
- Coverage     100%   99.6%   -0.4%     
========================================
  Files          55      56      +1     
  Lines        5673    5839    +166     
========================================
+ Hits         5673    5816    +143     
- Misses          0      23     +23

Impacted Files	Coverage Δ
pints/_mcmc/_slice_generalised_elliptical.py	87.34% <87.34%> (ø)
pints/_mcmc/_slice_stepout.py	99.28% <0%> (-0.72%)	:arrow_down:

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[MichaelClerx]

@ben18785 how's it going with all the slice samplers? I thought they were all nearly done but it seems to have slowed down