title: is there an EIGEN equivalent for gsl_matrix_isnull?
body: <p>Is there a function in Eigen for testing whether a matrix vanishes? Like gsl_matrix_isnull in GSL? I have searched for quite a while, but found nothing such.</p>
  
  <p>If not, is there anything faster than straightforwardly checking each element?</p>
  
  <p>Thank you and sorry if I overlooked it.</p>