photodoping approach (constrain DFT) or two chemical potential approach

Description

Here we introduce a constrained DFT approach for photodoping, where we take a few electrons from the valence to conduction bands. The electrons in the conduction bands stay there during the scf calculations.

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Photodoping approach (constrain DFT) or two chemical potentials approach

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.