RMG-database v2.0.0

In conjunction with the release of RMG-Py v2.0.0, an updated package for the RMG-database has also been released.
This release brings some new additions and fixes:

• Polycyclic thermochemistry estimation improved:
  • polycyclic database reorganized and more entries added in systematic way (input.thermo.groups.polycyclic)
• Database entry accessibility tests:
  • Fixes existing inaccessible entries in solvation/statmech/thermo of RMG-database