code: def _get_stddevs(self, coeffs, stddev_types):
          
          for stddev_type in stddev_types:
              assert stddev_type in self.DEFINED_FOR_STANDARD_DEVIATION_TYPES
  
          return np.sqrt(coeffs['sigma_bedrock']**2 + coeffs['sigma_site']**2)
docs: Equation (11) on p. 207 for total standard error at a given site:
  
          ``σ{ln(ε_site)} = sqrt(σ{ln(ε_br)}**2 + σ{ln(δ_site)}**2)``