A new instance of GradualDicomImporter for each file imported

GradualDicomImporter extracts data from incoming DICOM data, copies files into the appropriate location in the prearchive folder, creates prearchive session entries, etc. The problem is that a new instance of this is created for every incoming file. This includes retrieving the project data object, identifier, etc. Much of this is already cached, but could be much more efficient if applied to batches of files instead of each and every file.