Improvements on results (5): additional features for Neutron scattering

[mikibonacci]

• choosing the q for plots: for experimentalists is common to do not have the Gamma in the iBZ, but in outer zones. So, it can be useful to define the path by hands, like: (0,0,2) -> (0,0,3) .... and so on. Similar to what is done in bands but with vectors, not letters.
• Constant energy slice plots: plot the S(q,w) in a particular plane in the 3d q space. definition of the plane (3 q points) and of a maximum q distance. Or, define 4 points to detect a square. Definition of a mesh grid.
• make feasible to download the intensity maps: the json file seems to be to heavy...compress? or change filetype? The downloaded data should be not only phonopy but also the plotted quantities. #64
• Add also cm-1 to the units allowed in INS. also, allow change of units in phonon bands

To have examples on the output plots: https://journals.aps.org/prb/abstract/10.1103/PhysRevB.106.224304

[mikibonacci]

closed with #108