Add variance aggregates to data cube indexes.

[jheer]

• Add stddev, variance, covariance, and regression aggregates to data cube indexer.
• Add stddev, variance, covariance transforms in declarative specs.
• Add covariance helper to sql and vgplot APIs.
• Add column override support to Query.select.
• Add QueryManager logQueries method to toggle logging of queries submitted to connector.
• Add FilterGroup index method to toggle data cube indexing.
• Add advanced options for logging and optimization to the dev example runner.
• Add 10m point linear regression example.
• Fix bin transform to support extents for rule marks.
• Fix data type lookup in plot renderer.