Make EnsembleAtomGroup.ensemble() return original

[ALescoulie]

Quick fix for EnsembleAtomGroup making ensemble() method reference original.

[codecov]

Codecov Report

Merging #192 (61cfa64) into develop (5a9b22f) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #192      +/-   ##
===========================================
- Coverage    77.64%   77.60%   -0.05%     
===========================================
  Files           10       10              
  Lines         1606     1603       -3     
  Branches       227      226       -1     
===========================================
- Hits          1247     1244       -3     
  Misses         276      276              
  Partials        83       83              

Impacted Files	Coverage Δ
mdpow/analysis/ensemble.py	97.04% <100.00%> (-0.05%)	⬇️

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[orbeckst]

Turn the ensemble into a managed property (you can just apply my suggestions) and add a test for EnsembleAtomGroup.ensemble.

[orbeckst]

I applied the suggestion, but need to check the tests.