Rewrite of the viewer logic

[martin-schlipf]

The viewer should follow more closely the logic that is used for Graph. Specifically, the relevant py4vasp routines have a routine to_viewer and a wrapper plot that calls it. The viewer than contains all data required to produce the 3d view with attributes to set quantities.
Note that we should discuss specifics with Manuel. He might want to write a javascript library that generates the figures with Babylon or Three.JS directly. A preliminary step may be to rewrite the NGL interface but we should not invest to much time if the JS library gets developed.