added statisticNd, general code that should replace many functions, and sum for performance testing
added statisticNd, general code that should replace many functions, and sum for performance testing

maartenbreddels/vaex,maartenbreddels/vaex,maartenbreddels/vaex,maartenbreddels/vaex,maartenbreddels/vaex,maartenbreddels/vaex