docs: def getLaplaceCovar(self):
          """
          USES LAPLACE APPROXIMATION TO CALCULATE THE COVARIANCE MATRIX OF THE OPTIMIZED PARAMETERS
          """
          assert self.init,        'GP not initialised'
          assert self.fast==False, 'Not supported for fast implementation'
          
          if self.cache['Sigma']==None:
              self.cache['Sigma'] = SP.linalg.inv(self.getHessian())
          return self.cache['Sigma']