Add 'changed function implementation' detection

[petr-muller]

No description provided.

[coveralls]

Pull Request Test Coverage Report for Build 15

• 66 of 66 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 11:	0.0%
Covered Lines:	169
Relevant Lines:	169

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💛 - Coveralls

[codecov-io]

Codecov Report

Merging #2 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master     #2   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           3      3           
  Lines         115    169   +54     
=====================================
+ Hits          115    169   +54

Impacted Files	Coverage Δ
pyff/pyfference.py	100% <100%> (ø)	⬆️
pyff/pyff.py	100% <100%> (ø)	⬆️

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