BUG: Allow transformations with empty arrays

[snowman2]

• Closes Transformer.transform exception with empty inputs #766
• Tests added
• Fully documented, including history.rst for all changes and api/*.rst for new API

[codecov]

Codecov Report

Merging #768 (f57a0d2) into master (ba00b01) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #768   +/-   ##
=======================================
  Coverage   95.30%   95.30%           
=======================================
  Files          21       21           
  Lines        1513     1513           
=======================================
  Hits         1442     1442           
  Misses         71       71           

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