Updating thermo estimation documentation

[nickvandewiele]

As was requested by Bill, I will expand the current documentation on thermochemistry estimation in RMG-Py.

This is a list of updates I will include:

• [ ] Theory Guide
  • [ ] Thermochemistry Theory Guide
• [x] User Guide
  • [x] RMG User’s Guide
  • [x] Thermochemistry Estimation
  • [x] Databases
    • [x] Thermo Database
  • [x] Standalone Modules

Feel free to point out other additions that involve thermochemistry and that are not contained here above.

[nickvandewiele]

see #268.

For now, I will not expand the theory guide on thermochemistry.