Plot distribution of ancestor spans

[jeromekelleher]

For a given node u, its span is the width of genome where its a child (lets ignore trapped ancestral material)

child_edges = ts.edges_child == u
left = np.min(ts.edges_left[child_edges])
right = np.max(ts.edges_right[child_edges])
span = right - left

Plot the

1. Histogram for all non sample nodes
2. Scatter plot of span vs node time.

[savitakartik]

completed in #61