Issue 1004 unwrap

[richardjgowers]

Fixes #1004

Changes made in this Pull Request:

PR Checklist

• Tests?
• Docs?
• CHANGELOG updated?
• Issue raised/referenced?

[codecov]

Codecov Report

Merging #2012 into develop will decrease coverage by 0.11%.
The diff coverage is 11.53%.

@@             Coverage Diff             @@
##           develop    #2012      +/-   ##
===========================================
- Coverage     88.5%   88.39%   -0.12%     
===========================================
  Files          143      143              
  Lines        17249    17273      +24     
  Branches      2646     2656      +10     
===========================================
+ Hits         15266    15268       +2     
- Misses        1385     1408      +23     
+ Partials       598      597       -1

Impacted Files	Coverage Δ
package/MDAnalysis/core/groups.py	94.76% <11.53%> (-2.61%)	⬇️
package/MDAnalysis/lib/transformations.py	78.62% <0%> (+0.14%)	⬆️

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[orbeckst]

@richardjgowers is this PR still active?

The issue #1004 is still open and presumably, in the area of PBC/making whole we still have some work to do, at least from reading #1185 . Is this also your sense?

[richardjgowers]

@orbeckst yeah this PR is still relevant. Our current 'pbc' option equates to 'wrap', ie pack everything in before calculating an answer. What most people need is actually 'unwrap' ie make the molecule whole before giving me an answer. This PR would try and do that.