Changing name of "formation_energy" field in input files to better reflect what that term represents

[AdityaSavara]

As noted in the below pull request, the "formation_energy" field in the input file is actually the electronic energy contribution to the relative formation energy (it's not the full formation energy).

#149

Accordingly, the input file field should be titled E_formation_energy (or similar).

As noted in that pull request, this field name is expected to be updated to be more clear in the near future (between May 2022 and Jan 2024).