Firstorderstatisticstests

Kurtosis and Skewness are now initialized with Double.NaN since this is default MATLAB uses. 

The only other change is that VarianceIRT now has a higher priority than VarianceGeneric. VarianceGeneric returns the second central moment which should(!) be equivalent to the Variance but is just a little bit off. I think this is due to a rounding error caused by using double. But as a remark, this same error also exists in MATLAB,  the only reason I changed the priority is that we have a direct mapping of our features to the MATLAB ones (our Moment2AboutMean is equivalent to moment(A, 2) in MATLAB and our Variance is also equivalent to var(A)). 

If the MATLAB results should be verified don't forget to change the [display format](http://www.mathworks.de/de/help/matlab/ref/format.html). I personally used "format long g" so that the results aren't truncated.