backport of stub transplants of AtomGroup (PR #2427)

[orbeckst]

Fixes #1845 (for 1.x)

Changes made in this Pull Request:

• backport of PR Always transplant a stub for transplanted methods #2427
• install funcsigs package for Python 2.7 tests
• part of the 1.0.1 backport Release 1.0.1 #2768

PR Checklist

• Tests?
• Docs? – checked locally: methods are now included in AtomGroup
• CHANGELOG updated?
• Issue raised/referenced?

[orbeckst]

oops. missed a commit... stand by

[codecov]

Codecov Report

Merging #3007 into master will increase coverage by 0.71%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3007      +/-   ##
==========================================
+ Coverage   91.51%   92.23%   +0.71%     
==========================================
  Files         168      171       +3     
  Lines       22954    23618     +664     
  Branches     3186     3192       +6     
==========================================
+ Hits        21006    21783     +777     
- Misses       1322     1738     +416     
+ Partials      626       97     -529     

Impacted Files	Coverage Δ
package/MDAnalysis/core/groups.py	99.01% <100.00%> (+0.77%)	⬆️
package/MDAnalysis/core/topologyattrs.py	96.80% <100.00%> (+1.17%)	⬆️
package/MDAnalysis/topology/tpr/obj.py	96.82% <0.00%> (-3.18%)	⬇️
...onality_reduction/DimensionalityReductionMethod.py	97.14% <0.00%> (-2.86%)	⬇️
package/MDAnalysis/transformations/translate.py	97.43% <0.00%> (-2.57%)	⬇️
package/MDAnalysis/analysis/base.py	100.00% <0.00%> (ø)
package/MDAnalysis/lib/c_distances.pyx
package/MDAnalysis/analysis/encore/cutils.pyx
...alysis/analysis/encore/clustering/affinityprop.pyx
package/MDAnalysis/due.py	56.09% <0.00%> (ø)
... and 91 more

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