Fix NaN in JSON Feature

[ilkilic]

• Fix NaN values in JSON
• Ensure 'current' array operations use float64 to prevent casting error
• Refactor sAHP unit test
• Add APThreshold unit test

[codecov-commenter]

Codecov Report

Attention: 13 lines in your changes are missing coverage. Please review.

Comparison is base (0ae5960) 71.35% compared to head (d799ed0) 73.94%.

Files	Patch %	Lines
bluepyefe/ecode/DeHyperPol.py	0.00%	3 Missing :warning:
bluepyefe/ecode/HyperDePol.py	0.00%	3 Missing :warning:
bluepyefe/ecode/SpikeRec.py	0.00%	3 Missing :warning:
bluepyefe/ecode/negCheops.py	0.00%	1 Missing :warning:
bluepyefe/ecode/posCheops.py	0.00%	1 Missing :warning:
tests/ecode/test_apthresh.py	98.14%	1 Missing :warning:
tests/ecode/test_sahp.py	98.18%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #169 +/- ## ========================================== + Coverage 71.35% 73.94% +2.58% ========================================== Files 36 38 +2 Lines 2304 2391 +87 ========================================== + Hits 1644 1768 +124 + Misses 660 623 -37 ```

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