Fix transform stacking on multi-output aggregation

[gsheni]

• Fixes an assertion issue
• test case to check this.

[codecov]

Codecov Report

Merging #394 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #394      +/-   ##
==========================================
+ Coverage   95.97%   95.98%   +<.01%     
==========================================
  Files          91       91              
  Lines        7908     7916       +8     
==========================================
+ Hits         7590     7598       +8     
  Misses        318      318

Impacted Files	Coverage Δ
featuretools/synthesis/deep_feature_synthesis.py	94.5% <100%> (+0.01%)	⬆️
featuretools/feature_base/feature_base.py	96.22% <100%> (ø)	⬆️
...ols/tests/dfs_tests/test_deep_feature_synthesis.py	98.33% <100%> (+0.01%)	⬆️
...s/tests/primitive_tests/test_transform_features.py	98.05% <100%> (+0.01%)	⬆️

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[rwedge]

Can you also add a test case where DFS tries to stack features this way so that we hit the try-catch on codecov?

[gsheni]

@rwedge fixed with dfs test that uses a mutli output agg and transform