add None return for peak_times in calculate_feature() of extraFELFeature class

[darshanmandge]

To return None when the peak_times is None in calculate_feature function of extraFELFeature.

[AurelienJaquier]

@alejoe91 I'm pinging you to let you know we are making this change related to LFPy efeature

[alejoe91]

Thanks! Sounds fair :)

[codecov-commenter]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (6aa8937) 87.41% compared to head (12a4df9) 86.13%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #472 +/- ## ========================================== - Coverage 87.41% 86.13% -1.28% ========================================== Files 50 50 Lines 4330 4336 +6 ========================================== - Hits 3785 3735 -50 - Misses 545 601 +56 ``` | [Files](https://app.codecov.io/gh/BlueBrain/BluePyOpt/pull/472?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BlueBrain) | Coverage Δ | | |---|---|---| | [bluepyopt/ephys/efeatures.py](https://app.codecov.io/gh/BlueBrain/BluePyOpt/pull/472?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BlueBrain#diff-Ymx1ZXB5b3B0L2VwaHlzL2VmZWF0dXJlcy5weQ==) | `83.98% <14.28%> (-1.47%)` | :arrow_down: | ... and [5 files with indirect coverage changes](https://app.codecov.io/gh/BlueBrain/BluePyOpt/pull/472/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BlueBrain)

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