Changes in chemistry and initial guess of T

1. Barriers for surface reactions are included; previously not.
2. The part for H2 formation on grain is adjusted.
3. For the initial iteration use the temperature of the above cell as the
   initial guess.
4. Added one test module in disk.f90.
5. The hc_Tgas and hc_Tdust of heating_cooling module are set to be public.