Function frob_qsm_sq 

fn frob_qsm_sq(cfg: &GampCfg, der_1st: &[Complex64], weight_vect: &[f64]) -> f64

Frobenius norm squared of the weighted-dipole operator A_qsm.

||A_qsm||_F^2 = mut_cst^2 * ||g||^2 * sum_j |der_j * w_j|^2, where ||g||^2 = mean(D^2) (kernel_energy).