Redesign SimulatorControlItem and SimulatorControl

A SimulatorControlItem could be a Fix, Compute or a Variable. These come in the following types:

Not interesting (listed, but not any action)
Has a scalar value (value shown in the list, can be plotted as f(t))
Has multiple scalar values (can be plotted as f(t), multiple values in same plot such as pressure)
Has f(x; t) (can be plotted as f(x), possibly multiple values such as compute rdf)
Has per atom value [Can plot histogram and hovering will color each atom based on its value. Can have multiple values, so list needs to be shown)
Special type like fix indent

We should put properties on SimulatorControl so that GUI can be built up better. I have the following suggestions:

It should be possible to color all atoms based on per-atom value even when not hovering
For fix indent, it should be possible to always show the indenter (what happens if we have two or more?)
It should be possible to adjust frequency on the measurements (computes and variables only, not fix ave/time etc)

ovilab / atomify