code: def regularization_term(self):
          
          return np.matmul(self.solution_vector.T, np.matmul(self.regularization_matrix, self.solution_vector))
docs: Compute the regularization term of an inversion. This term represents the sum of the difference in flux \
          between every pair of neighboring pixels. This is computed as:
  
          s_T * H * s = solution_vector.T * regularization_matrix * solution_vector
  
          The term is referred to as *G_l* in Warren & Dye 2003, Nightingale & Dye 2015.
  
          The above works include the regularization_matrix coefficient (lambda) in this calculation. In PyAutoLens, \
          this is already in the regularization matrix and thus implicitly included in the matrix multiplication.