Modifications and tests
Added k-means Nyström package to lib

This is a package containing a highly precise Nyström implementation
with "k-means" sampling. Credits to Kai Zhang.

File exchange: Improved_Nystrom_Method
This calculates the inverse of the A matrix

This is used when self affinity is only removed for the S matrix, just
before evaluation of class labels. Doing this allows a much higher
precision and robustness for the Nyström method.

$A = I - \alpha \tilde{S}_diag$

Following Woodbury formula is used:
$(A + BCD)^{-1} = A^{-1} - A^{-1} B (I +CDA^{-1}B)^{-1} CDA^{-1}$
Rapidly faster affinity matrix calculation

Algorithm inspired/copied from `sqdist` in the *Improved_Nystrom_Method*
package.

Credits to **Roland Bunschoten**.

Modifications by me: Adjustments of the algorithms to make it calculate
the partial affinity matrix. We don't want the whole matrix using the
Nystroem method.