New plot type: Sankey diagram

[janosh]

We should add a Sankey diagram as a tool for visualizing space group distributions before vs after applying some symmetry breaking operation to a set of crystals.

[janosh]

Found another use case that made me implement this. Aflowlib and Spglib give different results for which symmetry operations apply to a given crystal structure. If you have a large dataset, Sankey diagrams are great for visualizing which spacegroups in spglib occasionally map to which other spacegroups in aflow.

Numbers before colons are international spacegroups, after colons are occurrence counts.