one-photon oep functional

Description

This is the implementation of the one-photon OEP functional of QEDFT described in Phys. Rev. Lett. 115, 093001 (2015) and ACS Photonics 5, 3, 992-1005 (2018)

News snippet

Implementation of the one-photon OEP functional of QEDFT described in Phys. Rev. Lett. 115, 093001 (2015) and ACS Photonics 5, 3, 992-1005 (2018)

Checklist

• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.