Add support for filled curves to ZedGraph package

[bruno-f-cruz]

There is currently no way to construct shaded areas with the plotting package. One seemingly obvious solution for this problem is to expose the Fill class from the ZedGraph library to the line plot in the bonsai library.

Documentation can be found here under "The Fill class"

While multiple constructors are available for the fill class, we should probably only expose the Fill( Color ) overload initially.