Add SpectralFunction property

[JosePizarro3]

One of the things I talked with Florian Dobener is that the SpectralFunction is different from the ElectronicBandStructure. The SpectralFunction.value is related with the intensity measured in ARPES or calculated from Green's functions (KKR, DMFT), $A(E_{k}, k)$, where $E_{k}$ are the eigenvalues at each point $k$ in the reciprocal space.

Visually, this is (note in the picture $\omega == E_{k}$):

The white line in this plot corresponds to ElectronicBandStructure.value, while the color map would be SpectralFunction.value. The relation between both is that the band structure is the delta Dirac version of the dressed-by-interactions spectral function.