Fixes transformation tests not running.

[Lp0lp]

Fixes #2241

Referenced in #2208

Changes made in this Pull Request:

• Added the missing __init__.py file in /testsuite/MDAnalysisTests/transformations/ that was preventing the transformation tests from running.

• Updated AUTHORS and CHANGELOG

PR Checklist

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

[codecov]

Codecov Report

Merging #2243 into develop will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           develop   #2243      +/-   ##
==========================================
+ Coverage     89.6%   89.6%   +<.01%     
==========================================
  Files          158     159       +1     
  Lines        19724   19728       +4     
  Branches      2780    2780              
==========================================
+ Hits         17673   17677       +4     
  Misses        1457    1457              
  Partials       594     594

Impacted Files	Coverage Δ
transformations/__init__.py	100% <0%> (ø)

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[orbeckst]

LGTM, thanks!