Error if relationship child variable is child entity index

[frances-h]

Error when adding an EntitySet relationship if the child variable is also the child entity's index
closes #1009

[codecov]

Codecov Report

Merging #1034 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1034   +/-   ##
=======================================
  Coverage   98.29%   98.30%           
=======================================
  Files         124      124           
  Lines       12587    12598   +11     
=======================================
+ Hits        12373    12384   +11     
  Misses        214      214           

Impacted Files	Coverage Δ
featuretools/entityset/entityset.py	97.65% <100.00%> (+0.01%)	⬆️
featuretools/tests/entityset_tests/test_es.py	99.85% <100.00%> (+<0.01%)	⬆️

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[rwedge]

Could this situation occur if the user passes entities and relationships to ft.dfs instead of calling add_relationship directly? If so, we might want a more descriptive error message.

[rwedge]

Looks good!