added 3D sample data. fixed a bug that was causing some issues distinguishing between 2D and 3D data. This is really an issue with the data file format, where the indices are different for 2D and 3D sets (this was from an eccentricity of XDMF/Visit), and should eventually be fixed on the data generation (MPACT) side. Also made the plot update when the slider is moved and added an option to use the global data scale for a 3D set to control the colormap.