Entangled units updates

[ddahlbom]

Fixes several bugs, including:
- Error is Base.show
- Error in initializing an external field from existing system
- Error in updating external field
- Sign convention for external field

Adds:
- Dispersion tests for dimer model
- Intensity golden test for dimer model using classical dynamics
- Additional function aliasing (clone_system, dispersion, resize_supercell)

[kbarros]

Overall looks good to me.

[ddahlbom]

In response to a request from Daniel Pajerowski, added Sunny.coherent_state_from_dipoles, a function that constructs a coherent state for an entangled unit based on a set of dipoles for each atom inside the entangled unit. These dipoles are all assumed to be fully polarized.

It is not possible to write a well-defined set_dipole! function for entangled systems, since all dipoles within a unit must be set at once. This function offers another path to initialize an entangled system in a classical dipolar state.

Note that it would be possible to write a batch set_dipoles! function for an entangled system, and perhaps this should be added. It would also be possible to set the state of entangled system from the coherent states of an SU(N) unentangled system, which could take the form of a set_coherents! function.