Using autodiff::real instead of ThermoScalar

[allanleal]

Let's use autodiff::real in ThermoFun (to enable automatic differentiation) instead of Reaktoro::ThermoScalar. This change will enable computation of derivatives with respect to virtually anything, and not just with respect to temperature and pressure (a limitation of ThermoScalar). For example, we'll be able to compute the derivative of a computed standard thermodynamic property (e.g., G0) with respect to model parameters (e.g., HKF coefficients).

This will also bring ThermoFun in sync with v2.0 of Reaktoro, which uses autodiff::real.

I'll be happy to help with the transition.