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 The formatting in statismo is not consistent which makes hard to read the code... Could you install universalindentgui, choose one style or set the preferred rules (if possible for astyle or uncrustify), please?
Then, we will reformat the whole library to make it consistent, then enforce each commit to format changes using hooks.