/**
  * Parametrized constructor.
  * \param aBelief The belief-state of the gaussian distribution.
  */
 gaussian_pdf(const BeliefState& aBelief) : mean_state(aBelief.get_mean_state()), E_inv(aBelief.get_covariance().get_inverse_matrix()), factor(-1) { 
   factor = determinant_Cholesky(E_inv);
   if(fabs(factor) < std::numeric_limits< scalar_type >::epsilon()) {
     factor = scalar_type(-1);
   } else {
     factor = scalar_type(1) / factor;
     for(size_type i = 0; i < E_inv.get_row_count(); ++i)
       factor *= scalar_type(6.28318530718);
   };
 };