add tracker module, moularize tracking system

corrlib/tracker.py

@@ -0,0 +1,22 @@
+import os
+from configparser import ConfigParser
+from .trackers import datalad as dl
+
+
+def get_tracker(path):
+    config_path = os.path.join(path, '.corrlib')
+    config = ConfigParser()
+    if os.path.exists(config_path):
+        config.read(config_path)
+    tracker = config.get('core', 'tracker', fallback='datalad')
+    return tracker
+
+
+def get_file(path, file):
+    tracker = get_tracker(path)
+    if tracker == 'datalad':
+        dl.get_file(path, file)
+    else:
+        raise ValueError(f"Tracker {tracker} is not supported.")
+    return
+