Ext potential vars recording

[Awallace3]

Description

Records the electronic-external potential energy and nuclear-external potential energy variables during an SCF calculation.

User API & Changelog headlines

• [x] Users can now access the wavefunction variables "ELECTRONIC-EXTERNAL POTENTIAL ENERGY" and "NUCLEAR-EXTERNAL POTENTIAL ENERGY"

Dev notes & details

• [x] The wavefunction variables "ELECTRONIC-EXTERNAL POTENTIAL ENERGY" has Vprime stored in ./psi4/src/psi4/libscf_solver/hf.cc and energy computed in ./psi4/driver/procrouting/scf_proc/scf_iterator.py once the final density matrix is acquired.
• [x] The "NUCLEAR-EXTERNAL POTENTIAL ENERGY" value is stored after being computed in ./psi4/src/psi4/libscf_solver/hf.cc

Checklist

• [x] Tests added for any new features
• [x] All or relevant fraction of full tests run

Status

• [x] Ready for review
• [ ] Ready for merge