Simplify heatmap examples

[RiboRings]

Hi!

Currently, the visualisation chapter shows heatmap examples with multiple packages (pheatmap, sechm and ggplot). These examples require a great deal of manual manipulation of the generated graphics to resize and combine them into a nice plot. However, virtually all the heatmaps in these examples can be generated with ComplexHeatmap in a simpler way. Thus, I propose to convert the current examples to use ComplexHeatmap instead.

Additionally, there could be a short comparison between ComplexHeatmap, suitable for hierarchical clustering, and sechm, suitable for rows ordered by the angles of an MDS plot (check issue #146).

I have streamlined and polished a little bit the code of the examples, but if anyone else is interested, they can carry on this work by forking or directly pushing to the ribo branch.