Fix small inconsistencies in ExcitedStates

[mfherbst]

I think there are a few inconsistencies in ExcitedStates, for example that we only allow using the length gauge for the plot_spectrum. Also the plot_spectrum function is getting a bit messy. I'll try to correct this here.

Some questions to @maxscheurer:

• What gauge is used for our rotatory strengths? Always velocity, right?
• Also do you have a good reference (paper, textbook) for the rotatory strength calculation. I think we should probably add one in the code.

[maxscheurer]

What gauge is used for our rotatory strengths? Always velocity, right?

Yes, in adcc we only use velocity gauge.

Also do you have a good reference (paper, textbook) for the rotatory strength calculation

We should ask @Drrehn, he's the expert. I guess it's somewhere in the Molecular Properties book?