Processes a single quantity as described and used
        in `scipy_data_fitting.Fit.computed_quantities`.

def compute_quantity(self, quantity):
        """
        Processes a single quantity as described and used
        in `scipy_data_fitting.Fit.computed_quantities`.
        """
        quantity = quantity.copy()
        expression = self.model.replace(quantity.pop('expression'), self.replacements)
        variables = self.all_variables[len(self.free_variables) + 1:]
        function = self.model.lambdify(expression, variables, **self.lambdify_options)
        quantity['value'] = function(*(self.fitted_parameters + self.fixed_values)) * prefix_factor(quantity)**(-1)
        return quantity