First release after the package being published in Nat Meth

We create the first official release of the kBET package after its publication in Nat Meth in January 2019. Changes in the package are minor (full backward compatibility in almost all cases). Changes apply to treating missing values in knn-graphs (ignore NA values). Further, in case of small neighbourhoods, kBET returns a warning (instead of an error message) and NA as result.