Improvements to `QuickPlot`

QuickPlot could do with a few updates. Feel free to add more to this list:

• More obvious error when two variables have different dimensions
• More obvious error when axlims are inf
• Allow plotting a single 3D variable (contour plot)
• Make the x-axis label “x” when the spatial scale isn’t found (or raise error?)
• Should there be default output variables? Or should they always have to be passed in (which then makes it more obvious how to change them?)
• Try to clean up set_output_variables to make it more obvious what is being done
• Slider sometimes obscures the figures
• Should we be calling it SliderPlot?

[rtimms]

can we just move isnotebook to QuickPlot so we can use it directly in notebooks without having to do the slider widget manually?

[Scottmar93]

One additional thing that would be nice, is if QuickPlot can accept solution objects and simulation objects. If a simulation object is passed, it just does simulation.solution automatically. I am finding myself writing [sim1.solution, sim2.solution, etc] or having to do [sim.solution for sim in [sim1, sim2, sim3]] quite a bit, which is just messy