Refinement support

[lkotipal]

• Adds support for plotting the alpha_2 criterion based on J/B_perp
• Adds support for contour plotting refinement level in grayscale along something else
• Change parameter lin of plot_colormap3dslice to be the amount of tickmarks for linear scale. lin=True uses the old default of 7.

[markusbattarbee]

Now that Markku implemented the new ability to pass the reader to datareducers, let's use that method instead here as well.

[lkotipal]

Done