[doc] updated the user manual : added get/setPDFPrecision methods, getLastPDFError and getCorrelation methods
[src] updated computePDF: added some checks on the matrix and optimization when matrix is square
[test] added new test R^2-->R^2 for the validation of optimization added
[test] added new test R^2-->R^2 result
[test] updated R^2-->R^2 analytical test
[test] updated pdf results