Make contour lines and caps intelligent

[RachelAlcraft]

The contour lines are sometimes so steep they look like a colour, and the cap on density hue to enhance the plot should be intelligent as for cryo-em structures it doesn't enahnce enough. Additionally add a user option to slide the contour cap and maybe the lines.

[RachelAlcraft]

For heatmaps the solution is probably somewhere here: https://discourse.julialang.org/t/discrete-colorbar-with-plotlyjs/80773/2