Design documentation

Happy new year.

As briefly discussed prior to the holidays here is a start to document the design. The documentation for the XML should be sufficient to jolt our memory in the future and help people that are interested in contributing to get started.

The diagram of the overall data-flow/functionality just shows the very top level at this point, more to be added in the future. Adding information, i.e. expanding the diagram and writing up some text about the data-flow and the code organization will be easier as we start restructuring once the XML is getting switched over to the "new" data structure.