xTB: Support parsing of more auxiliary files

[Andrew-S-Rosen]

We should support parsing of additional xTB files. This includes but is not limited to:

• xtbout.json: This contains JSON-formatted data that is either higher precision or not present at all in the log file. The file is only written with the --json flag. For instance, this files contains all of the orbital energies, whereas this information is truncated in the log file. Additionally, it always contains the number of unpaired electrons, which is not always present in the log file.
• xtbopt.log: This contains the SCF energies and structures as a function of iteration, which will be higher precision than the printed log file.
• xtbopt.xyz: Contains the final structure.
• hessian: Contains the hessian data. I actually already made the parser here, but it crashes when used because it reaches EOF.