WIP: Added the ionic contribution to the polarization-induced gauge-field propagation.
This feature is under work in progress. Please do not merge it.
- I added the ionic contribution to the gauge field propagation. At the moment, only the electronic contribution is considered in the Maxwell equation, but the ionic component is just ignored.
- I added the gauge field contribution to forces acting on ions.
- I have checked that my code follows the Octopus coding standards
- I have added tests for all the new features added in this request.