add support for data matrices already centered about zero

While using this library for its PCA functionality, I needed to be able to customize the PCA space by inputting a data matrix already centered about mean zero.

This makes it equivalent to calling the function prcomp() in R with the variable center=FALSE. It adds functionality to the original library.

This is not a perfect fix, it still computes the means when it doesn't need to. But, it adds functionality and does not affect previous functionality, either.