Change fSys datastructure

Created by: scarrazza

Issue by Zaharid Sunday Jul 02, 2017 at 10:18 GMT Originally opened as https://github.com/NNPDF/libnnpdf/issues/25

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Experiment should get a packed array (as in vector<double> buffer of unique_ptr<double*> buffer) of correlates systematics and another packed array of uncorrelated ones at construction time. We shouldn't have to dereference a deeply nested structure in a tight loop to query basic information, which is furthermore duplicated since the systematic types are the same for each point.