Introduced separate class for Nystrom method.

The Nystrom method is interesting in its own right, and not only within the context of the LowRankGPModelBuilder. We have disentangled the logic of the low-rank approximation from that of the Nystrom method and put the code for computing the Nystrom method into its own class.
@marcelluethi astyle is easier to configure (as long as your style is not too far from standard formatting), but uncrustify is much more advanced (you can basically control anything). The most important part now is to choose the style you'd like to impose to the project.