fit null model

    Y       NxP phenotype matrix
    S_XX    eigenvalues of the relatedness matrix
    U_XX    eigen vectors of the relatedness matrix

def fit_null(Y,S_XX,U_XX,nfile,F):
    """
    fit null model

    Y       NxP phenotype matrix
    S_XX    eigenvalues of the relatedness matrix
    U_XX    eigen vectors of the relatedness matrix
    """
    mtSet = limix.MTSet(Y, S_R=S_XX, U_R=U_XX, F=F)

    RV = mtSet.fitNull(cache=False)
    params = np.array([RV['params0_g'],RV['params0_n']])
    np.savetxt(nfile+'.p0',params)
    np.savetxt(nfile+'.nll0',RV['NLL0'])
    np.savetxt(nfile+'.cg0',RV['Cg'])
    np.savetxt(nfile+'.cn0',RV['Cn'])