In tl.clonal_expansion replace `clip_at` with `breakpoints`

[grst]

For datasets with higher clonal expansion, it would be great to define custom breakpoints (e.g. 1,2,5,10,20,50) for clonal expansion rather than the 1,2,3,...,>=n that is currently possible with clip_at.

[grst]

Note that the corresponding group_abundance then needs to natsort the legend. Show an example with viridis colorscale.

Maybe combine fixing this with #232