Big change is going to be made.

When solving temperature from the heating/cooling balance, every time
T is changed, a new solution of chemical abundance will be given.
Small change

1. The 0.5*dNcol thing is got rid off.
2. The attempt to calculate the "real-time" chemistry during the solution of T
   is temporarily given up.
Small changes.

1. Gas-dust collision cooling formula taken from AGN paper.
2. Add the geometric factor of X-ray and UV to configuration file.
Output N_H2, N_H2O, etc.

1. Going to add grid refinement based on current whole-disk results.

2. Maybe also going to treat Tgas as a time-dependent variable and solve the
   thermal balance together with the chemical evolution.