Feature StetsonJ return unexpected value

[leliel12]

The extractor feets.extractors.ext_stetson.StetsonJ documentation says

For a Gaussian magnitude distribution, J should take a value close to zero.

but the real result is of the feature is ~-0.41

This test is implemented in this test: https://github.com/carpyncho/feets/blob/40157f9bbfad87cc23753bad76a8414614e56814/feets/tests/test_extractors.py#L197-L209

[leliel12]

When you require this feature or anyone from the issues #2 #3 #4 you get a ExtractorWarning

Python 2.7.6 (default, Oct 26 2016, 20:30:19) 
[GCC 4.8.4] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import feets
>>> feets.FeatureSpace(only=["AndersonDarling", "StetsonK", "StetsonJ"])
feets/extractors/core.py:227: ExtractorWarning: The original FATS documentation says that the result of StetsonK must be 2/pi=0.798 for gausian distribution but the result is ~0.2
  warnings.warn(w, ExtractorWarning)
feets/extractors/core.py:227: ExtractorWarning: The original FATS documentation says that the result of AndersonDarling must be ~0.25 for gausian distribution but the  result is ~-0.60
  warnings.warn(w, ExtractorWarning)
feets/extractors/core.py:227: ExtractorWarning: The original FATS documentation says that the result of StetsonJ must be ~0 for gausian distribution but the result is ~-0.41
  warnings.warn(w, ExtractorWarning)
<FeatureSpace: AndersonDarling(), StetsonK(), StetsonJ()>
>>>