Update the array chunker to extract all support attribute types from SciDB.
Models can store any supported type in an array.  Added a scatterplot index artifact to the CCA model.
The get_array_attribute() helper function handles all supported array attribute types.
CCA model scatterplot uses a list of explicit observation indices for selections.
Correct an error converting arraybuffers with offsets.
Use global indices to color the CCA scatterplot.
Filter NaNs when when we compute the CCA model, instead of when we import the data.