Implement working Datacite data source (WIP)

- Re-implement VTSource, TarFiles, TarFilesCursor, WorksCursor
- Pass code through black

The following now work:
bin/a3k  query datacite tests/data/datacite.tar.gz \
  -q 'SELECT id,container_id,doi,url FROM Works'
bin/a3k query datacite tests/data/datacite.tar.gz \
  -q 'SELECT id,container_id,work_id,name FROM work_creators'
bin/a3k query datacite tests/data/datacite.tar.gz \
  -q 'SELECT works.id,works.container_id,doi,name FROM works
    INNER JOIN work_creators ON work_creators.work_id = works.id'

Still needed:
- Pass pylint
- Unit and integration tests
- Refactor duplicate of progress bar
- Examples

src/alexandria3k/data_source.py

@@ -260,7 +260,8 @@ def Column(self, col):
 
     def Close(self):
         """Cursor's destructor, used for cleanup"""
-        self.parent_cursor.Close()
+        if self.parent_cursor:
+            self.parent_cursor.Close()
         self.elements = None