2D genomic heatmaps

[eweitz]

This adds a new annotation layout for 2D genomic heatmaps.

Such 2D heatmaps are great for visualizing large data tables. The motivating example for 2D heatmaps is depicting an expression matrix of how much each gene is expressed in each cell -- for all genes across all cells in a sample set. (Expression matrices are commonly output from single-cell RNA-seq.) The rich granular views from 2D heatmaps complement the glanceable summaries provided by 1D heatmaps.

The 2D heatmap requires a TSV expression matrix file, and can only be displayed vertically on a single human chromosome at a given time. Support for more organisms and more efficient matrix file formats is planned.

This brain cancer example shows how 2D heatmaps enable inspection of genome-wide summaries:

[coveralls]

Coverage decreased (-0.2%) to 91.605% when pulling 6bdccd7 on 2d-genomic-heatmaps into 770ce1f on master.