ENH: adds hook for loading directory archetypes

When a directory is added via the Add Data dialog, it may contain a collection of files that should be treated as a single MRML data type, such as a series of image files that get loaded as a volume.

This change allows a qSlicerFileReader subclass to define a method that can examine the contents of a directory and filter out any files that should be loaded as a group. One of these files then serves as the archetype and the io properties can be configured such that the files are loaded correctly.