Update of mlab.pca - updated docstring, added saving the eigenvalues.

The old version of pca class did not save the Eigenvalues - just the "fractions" that are just percentage values.

Also added info which row of Wt one can find the Eigenvectors.
Merge pull request #2239 from blackw1ng/patch-1

Update of mlab.pca - updated docstring, added saving the eigenvalues.