Improve visualization of large heatmaps by interpolation

[cnluzon]

I ran into an example where a pattern that exists in the heatmap is not clearly seen unless the heatmap is saved in a very large image. The current subsampling of rows is not sophisticated enough to show these patterns if the data is a bit noisy. The same plot using deepTools looks clearer, and it seems that they use interpolation when visualizing the heatmap.