ENH: Set Slicer DICOM database as default output directory for export

The default output directory is set to the Slicer DICOM database directory. If the selected directory is still a Slicer DICOM database, then the files are exported in its dicom subfolder (create if does not exist)
ENH: Scalar volume export to DICOM works

DICOMDataExchange split into DICOMExportScalarVolume and DICOMExportScene, because now they are two separate operations, and they do not use common utility functions any more; All tags are saved to the exportable so that the DICOM plugin can get the patient and study tags too; Patient and study tags are stored in vtkMRMLSubjectHierarchyConstants.h in a way that they can be queried by tag, attribute value, or list of tags for each level; If DICOM database folder (default) is selected for export, then the series is exported to a temporary folder, then moved to the database folder by the indexer, finally the temporary folder is deleted; Exported series is added to the database after successful export.