Make standardization of input optional in mlab.PCA

In principal component analysis, standardization of input data should be a user choice, depending on the problem at hand. The current version always standardizes, without offering a choice to the user. Therefore, I added an option to the PCA class where the user can specify whether the input data are to be standardized before performing the PCA. When *standardize* is set to False, only centering is performed, without dividing by sigma. The default is to standardize, to remain compatible with the current version.