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.
minor edits
Minor style/performance improvements.
minor fix: explicit cast from size_t to float
Merge branch 'develop' into factoredout_nystrom

Conflicts:
	modules/core/include/LowRankGPModelBuilder.h
fixed more cast issues
fix issues from Arnaud's review
Merge branch 'factoredout_nystrom' of github.com:statismo/statismo into factoredout_nystrom

Conflicts:
	modules/core/include/LowRankGPModelBuilder.h
Merge pull request #93 from statismo/factoredout_nystrom

Introduced separate class for Nystrom method.