refactored the datafile classes to do proper inheritance and to treat the access functions as honest-to-goodness virtual functions. Added an OpenDataFile routine to resolve file type and instantiate the right derived class for the actual file. also fixed a minor bug in the get_data_info function for the Sn Vis data file type which wasnt returning the right number of planes for 2D data. fixed the axial slider class to disable itself if there is only one plane in the plotted dataset. moved composite file functionality into the Sn vis class, since it is specific to that file format.