ENH: Add energies consistent with LAMMPS to Tersoff

This MR is on top of !3655 (merged) and marked as draft until that is merged.

This MR adds energies consistent with LAMMPS to Tersoff.

Checklist: x (done) or ~ (irrelevant)

• I am familiar with ASE's contribution guidelines.
• Doc strings in code changed in this MR are up to date.
• Unit tests have been added for new or changed code.
• Changes have been added in changelog.d using scriv.
• Issue is resolved via "closes #XXXX" if applicable.