Compute Negative Eigen Fraction from PE dataset or mapping
[F,R] = NEF(W)
The negative eigen-fraction is a ratio expressing the sum of magnitudes of all negative eigenvalues divided by the sum of magnitudes of all eigenvalues. The necessary numbers are stored in W = PE_EM(D) or in A = D*W, in which D is a symmetric dissimilarity matrix.
The negative eigen-fraction is the ratio of the magnitude of the lowest negative eigenvalue and the largest positive eigenvalue.