docs: 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'])