Calling kurtosis correctly with SD instead of variance

`DoubleDescriptive/kurtosis` was getting called with variance instead of standard deviation.

After changing this to use `sd` it returns the correct value for data from #226, wich is  -0.11735294117647133. The same value gets calculated with R package e1071, using the fourth central moment and sample standard deviation formula (see http://www.r-tutor.com/elementary-statistics/numerical-measures/kurtosis).

Furthermore `kurtosis` is invariant against multiplication with a constant.

Both scenarios were added as tests.

Fixes github issue #226