[BugFix] Dictionary metric

[Mv77]

There is a bug in the distance metric for dictionaries.

This PR fixes it and adds a test.

• Tests for new functionality/models or Tests to reproduce the bug-fix in code.
• Updated documentation of features that add new functionality.
• Update CHANGELOG.md with major/minor changes.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.07 🎉

Comparison is base (39b52bb) 73.20% compared to head (801c57e) 73.27%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1260      +/-   ##
==========================================
+ Coverage   73.20%   73.27%   +0.07%     
==========================================
  Files          78       78              
  Lines       12821    12827       +6     
==========================================
+ Hits         9385     9399      +14     
+ Misses       3436     3428       -8     

Impacted Files	Coverage Δ
HARK/metric.py	89.58% <100.00%> (+16.66%)	⬆️
HARK/tests/test_core.py	100.00% <100.00%> (ø)

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[alanlujan91]

Great catch!