ENH: Sank addSeriesInSubjectHierarchy function down to abstract DICOMPlugin

The addSeriesInSubjectHierarchy function (which was in DICOMScalarVolumePlugin so far) adds the loaded node to subject hierarchy to the proper patient (subject) and study according to the DICOM tags from the first file referenced in the associated loadable. This function is now accessible from all the DICOM plugins. It needs to be called after the loaded object is created in the load() function. Now it is called by DICOMDiffusionVolumePlugin too.
ENH: vtkMRMLHierarchyNode methods made virtual

Two methods in vtkMRMLHierarchyNode made virtual. GetAssociatedChildrendNodes contained a typo so it is now called GetAssociatedChildrenNodes. Subject hierarchy volumes plugin now can add volumes from potential list to tree