Visualization: Callback, extensions etc.

[ranocha]

Use this issue to track open questions/issues related to the new visualization approach of plotting directly from the REPL:

• [x] Document capabilities for 2D slices of 3D data (done in #403)
• [x] Continuing #350, it would be nice to have a callback supporting visualization on the fly while running a simulation (done in #403)
• [x] 1D plots (tracked in #198)
  • [x] general plotting
  • [x] plot from 2D/3D
• [x] Rewrite and consolidate current approach
• [ ] Plot on arbitrary points to, e.g., support plotting values along a semicirlce
• [ ] Support plotting for structured curved mesh
  • [x] 2D
  • [ ] 3D
• [x] Think about how to support Makie
• [ ] Reading solution/restart files for plotting
• [ ] Possibilities for batch processing of HDF5 files
• [ ] Extend to plotting of semidiscretization + coefficients (and document that part)
• [ ] Document how to access (and work with) uniformly spaced data from PlotData2D etc.
• [ ] How to handle the dependency on the Plots (and possibly other) package(s) (see also https://github.com/trixi-framework/Trixi.jl/pull/403#discussion_r549679762)?
• [ ] Optimize plotting performance (especially mesh plotting becomes unbearably slow when the number of leaf cells increases)
• [ ] Movie callbacks (https://github.com/trixi-framework/Trixi.jl/issues/842)
• [ ] Flexible PlotDataX axes (https://github.com/trixi-framework/Trixi.jl/issues/590)
• [ ] In-situ 3D plotting (https://github.com/trixi-framework/Trixi.jl/issues/859)

[FelipeSantillan]

559 and #609 enable 1D plots from 2D/3D solutions.

[jlchan]

I've updated this to consolidate some visualization issues, as discussed in the Trixi meeting.