Statistics: DescriptiveStatistics Kurtosis fix

Hello, I was using MathNet library and suddenly found an issue:
DescriptiveStatistics computes kurtosis incorrectly for large series (my example is a sample of Meixner(0,Sqrt(2),0,1) of 10000 values).

The proof (as a test) and the fix is already in my branch: [statistics-kurtosis](https://github.com/ArtyomBaranovskiy/mathnet-numerics/tree/statistics-kurtosis)

Also there is a small fix for parsing double int StatTestData for machines which have not an English locale as default

Short commit history:
Fix parse double exception for non-english locales as default;
Implement proof (test) that the former formula for computing Kurtosis workes incorrectly;
Fix Kurtosis in DescriptiveStatistics class