plot_colormap3dslice overlays fSaved, fluxrope and AMR levels

[ykempf]

Now done with straight cell edges instead of interpolated contours. Example:

[markusbattarbee]

I think this would be a good place to add some modularity, i.e. make a function which takes the data, contour points, linestyles and axes and then does this by-cell-edges plotting. Then we could re-use it for 2D plotting tools as well, as well as threeslice etc. (and include a check inside that function to return without doing anything if there isn't any points passing that contour value).