Fix Parent attribute handling for features with multiple parents

[standage]

The solution to #84 was pretty simple: adding append=True to the child's Parent reset logic when a feature's ID is updated.

Evaluating a new failure in the test suite revealed an error in one of the test data files. This error has also be corrected.

Fixes #84.

• Full test coverage? Any additional tests that could easily be added?
• API documentation up-to-date? Docstring examples?
• Is the CHANGELOG up-to-date?

[codecov]

Codecov Report

Merging #85 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #85   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           25        25           
  Lines         1423      1422    -1     
  Branches       313       313           
=========================================
- Hits          1423      1422    -1     

Impacted Files	Coverage Δ
tag/feature.py	100.00% <100.00%> (ø)

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