class ROOT::Minuit2::FunctionMinimizer

 base class for function minimizers; user may give FCN or FCN with Gradient,
    Parameter starting values and initial Error guess (sigma) (or "step size"),
    or Parameter starting values and initial covariance matrix;
    covariance matrix is stored in Upper triangular packed storage format,
    e.g. the Elements in the array are arranged like
    {a(0,0), a(0,1), a(1,1), a(0,2), a(1,2), a(2,2), ...},
    the size is nrow*(nrow+1)/2 (see also MnUserCovariance.h);