Add 1D histograms to the heatmap axes

[esheehan-gsl]

Add a supplemental display to the heatmap that shows the distribution for each axis long that axis on the opposite side from the axis labels. We’ll need to be able to hide the annotations and one of the axes on the histograms; they should share an axis with the heatmap, so they don’t need to reproduce the same labels. Brushing should be supported on each histogram as well as the heatmap, and the selection indicator should update on all relevant axes.