Expose formation energies
A user has requested to revive the API for formation energies. The reason it was removed was to prevent misuse of the info (e.g. combining values from equilibrator with other formation energy tables, which would be incompatible and likely violate the 1st law). On the other hand, some models such as thermodynamic-based sampling of the flux space can be more straightforward using formation energies rather than reaction energies.