Marshal _indexToPoints to interaction events; other additions

[monfera]

We have added logic to transforms such that the ultimate results of transforms can be associated with the original data points, via their positional indices. This way, interacting with e.g. an aggregate bar, the original points can be known, which is crucial for crossfilter to work, among other possible uses.

The linked PR took care of the logic but exposing it via events is a useful next step: #2126 (review)

Besides this, other changes are needed:

• cover groupByand sort transforms as well (groupBy is not currently working with crossfilter, so it was less of a priority) done in More indexToPoints  #2162
• extract the few lines that are identical between the filter.js and aggregation.js implementations
• add more test cases with various transforms sequences

[monfera]

Regarding the implementation, how @alexcjohnson proposes it above seems to be the best way.

[etpinard]

Resolving this would fix #1807 I believe.

[etpinard]

as only item

add more test cases with various transforms sequences

is left, changing the label from feature to maintenance.