Initial commit
Works! Datasets with different means, sds and ns are now aggregated.
After http://books.google.de/books?id=cn3JtP5ugQsC&lpg=PA141&ots=YHV9PQJB9v&dq=varianz%20von%20%20zusammengefasste%20gruppen%20formel&hl=de&pg=PA141#v=onepage&q&f=false