Fix reading core nodes file while in different cwd

src/main.py

@@ -1,6 +1,7 @@
 # -*- coding: utf-8 -*-
 import csv
 import logging
+import os
 import pickle
 import socket
 
@@ -27,7 +28,7 @@ def main():
         logging.warning('Error while loading nodes from disk.')
         logging.warning(e)
 
-    with open('core_nodes.csv', mode='r', newline='') as f:
+    with open(os.path.dirname(os.path.realpath(__file__)) + '/core_nodes.csv', mode='r', newline='') as f:
         reader = csv.reader(f)
         shared.core_nodes = {tuple(row) for row in reader}
         shared.node_pool.update(shared.core_nodes)