ecodes with amp2 output fix

[AurelienJaquier]

• Fixes output of DeHyperPol, HyperDePol and negCheops so that thresh_perc represent the amplitude that is not fixed
• Add some docstrings to ecodes better describing the ecodes
• edge case for mean of amp2 with relative amplitude

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (dfa0d83) 66.00% compared to head (0051d44) 65.95%. Report is 1 commits behind head on master.

Files	Patch %	Lines
bluepyefe/protocol.py	33.33%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #161 +/- ## ========================================== - Coverage 66.00% 65.95% -0.05% ========================================== Files 34 34 Lines 2156 2159 +3 ========================================== + Hits 1423 1424 +1 - Misses 733 735 +2 ```

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