Plot quantities like pressure

[efaulhaber]

So far, we can only plot all particles of a system with a constant color in Plots.jl. The next step is to add different quantities.
Maybe something like

plot(sol, c=pressure)

where pressure is a function exported by TrixiParticles.jl? Idk if that is possible.
Or the way it is done in Trixi.jl. By default, you get a plot of all quantities:

Then, you can work with PlotData2D structs to plot a single quantity:

Since this is not trivial, and the API is not clear yet, I suggest we do this in 0.2. @LasNikas @svchb ?

[efaulhaber]

The following would be easy to implement, but not very flexible/modular in the long run:

plot(sol, quantity=:pressure)

Then we only support a fixed set of quantities, and it's not clear what happens in multiphysics simulations when we have different systems. But at least we have a (very experimental) way to easily plot pressure, density and so on.

[LasNikas]

I'm fine with doing this in 0.2. IMO, plotting of quantities is usually done in paraview.