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.
Fix #2423: Clip markers off figure.
Fixes issue #2525: clears _suptitle instance on clf() and fixes a bug
where suptitle endlessly adds text artists to the figure
Merge pull request #2711 from cimarronm/figure_suptitle_fix

Remove suptitle on clf.
Merge pull request #2239 from blackw1ng/patch-1

Update of mlab.pca - updated docstring, added saving the eigenvalues.
Merge pull request #2423 from mdboom/cull-off-of-figure-markers

Off-axes markers unnecessarily saved to PDF