updated notes with workflow simplification tasks
switched workflow to a static plan, each operator consumes and generates nested arrays of data
added explicit port mappings between nodes during flow.instantiate.  all examples run
categorical and numeric stacking work for all shapes
refactored stacking code
commit before making xform extend node
xform extends gg.wf.Node.  Client/server split execution running correctly for all existing specs.  Server side layout algs may not estimate dom element sizes accurately
fixed axis/facet label positioning.
updated notes
readme