Thermo for training reactions in reverse

If training reactions are written in reverse, we use thermo to fit the kinetics in the forward direction. If the thermo is poor, the rate we train the kinetics tree with will be poor as well.

For example:
Training reaction 2 in Birad_R_Recombination NO2_p <=> NO + O is in reverse direction. To reverse it, we need to get the thermo for NO2, NO, and O. O is in the primaryThermoLibrary, but NO and NO2 are not. If NO and NO2 are not in any of the thermo libraries specified in the input file when running RMG, then RMG uses group additivity to estimate the thermo, and since these estimates are not so good, the kinetics will be not so good. Therefore, even if we are running RMG without nitrogen in our system, we need good thermo for NO and NO2, otherwise the Birad_R_Recombination tree might be trained with poor kinetics.

Possible solutions:

• Only write training reactions in the forward direction. That way, our RMG model will not be dependent on the thermo for the species in reverse training reactions.
• Put all the species in all the reverse training reactions into a "Training_reactions" thermo library and load this library by default every time rmg is run.

Start by reading the issue's 11-comment discussion and tracing how reverse training reactions obtain thermo data and how configured thermo libraries are loaded. No file or test is named in the issue; done would require an agreed approach that prevents poor thermo from degrading kinetics for reverse training reactions, with regression coverage.