Added Matrix and HeatMap View types and plotting code

The Matrix View type was added as a mix-in baseclass for SheetView
and HeatMap. All three are now plotted via the MatrixPlot plotting
class. While Matrix and SheetView are for dense and linear data,
HeatMap supports non-linear sparse data.
Fix to aspect ratio calculation to allow integer handling
Updated doc/reference_data submodule reference
Reverted Collector capitalized attribute changes
Fixed Matrix docstrings
Added Matrix and HeatMap test comparisons