Provide a way to specify an index for sorting slices in a pie or sunburst chart.

This should be useful in the following use cases:

• we want to treat one particular slice differently from the other ones, like the Other slice that we'd like to render as the last slice, even if its value is larger than some of the category-assigned slices
• during animation (eg. time playthrough, or paging back/forth among category filters) for visual stability between data updates, avoiding slices rotating over one another
• to use in particular cases where the order is implied by the semantics of the categories, such as data that is ordinal in its meaning (e.g. perfect, good, ok, bad)