Dipole moment writer for new RTTDDFT

Closes #1433. This MR

• Fixes the extra vacuum and moment correction Poisson solvers to take the xp argument (otherwise RTTDDFT breaks)
• Introduces a dipole moment writer for new RTTDDFT. It is very similar to the old dipole moment writer. But I figured we can remove the center and density_type options. The dipole moment writer needs to be called in the ipropagate loop with the RTTDDFT history, the state, and the potential calculator. From the history, the current time, and previous kicks, are extracted. If one or several kicks were just performed, then comment lines are written correspondingly.
• Updates the documentation correspondingly.
• Adds a unit test of the new dipole moment writer. The test is super fast so I added it to the standard CI.
• Removes the RTTDDFTResult. Now the ipropagate iterator just yields the current time. The result would only be useful for the dipole moment writer, but not any other writer. I think it's best to keep it simply.