Expand derivatives of array variables

[hersle]

Fixes #1383 with @willrae42's solution.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.

Project coverage is 8.78%. Comparing base (9df148a) to head (af168c3).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/diff.jl	0.00%	8 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff            @@
##           master   #1469      +/-   ##
=========================================
- Coverage    8.79%   8.78%   -0.02%     
=========================================
  Files          51      51              
  Lines        4924    4931       +7     
=========================================
  Hits          433     433              
- Misses       4491    4498       +7     

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