Rewrite plotting functions and include introduction notebook

[pjaap]

This is a major rewrite of the plotting functions defined in ct_plotting.jl.

@mariekeosw and I changed the following things (cf. CHANGELOG entry):

• all library plotting functions in ct_plotting are now changed to use a GridVisualizer

Before, we had a hard dependency on PyPlot which caused spurious problems on some systems. Now a Plotter of choice can be used via GridVisualize.jl.

• the old signature is still available but deprecated

Otherwise the version would be breaking. However, the user needs to add PyPlot of PythonPlot to their global enviroment to use the old function.

• the user can choose any plotter supported by GridVisualize (currently PythonPlot, Makie flavors and PlutoVista)
• PyPlot and Plots are also supported, but not recommended
• in the examples, the plotting key word is removed. No plotting is indicated by Plotter = nothing
• all 1D examples are rewritten to use the new plotting routines
• all 2D examples are still hard-wired to Py[thon]Plot, but the users have to provide this package in their own (global) environment

The 2D examples make heavy use of Matplotlib features and this should be kept as is.

---

This change allows also to generate composed images in the examples instead of generating a single image for each plot.

TODOs:

• Remove the activate line in the notebook
• use PlutoVista as Plotter in the notebook, this is currently broken by a weird JS problem: Plotly.js somehow breaks LaTeX rendering JuliaPluto/Pluto.jl#3439
• symmetric log scaling in Makie backends (used in doping plots) works, but displays poor ticks: pseudolog10 doesn't give nice ticks MakieOrg/Makie.jl#1319
• PlutoVista does not support symlog scaling so far

[pjaap]

Ok, I'll remove the activate line from the notebook and merge