Improve resource loading: do not rely on correct working directory

Resources will now be loaded from an absolute path,
which was derived from the location of the current
module.

tests/test_consistency.py

@@ -9,7 +9,7 @@ from stw_potsdam.config import read_canteen_config
 
 
 def _resource_path(filename):
-    return os.path.join('tests', 'resources', filename)
+    return os.path.join(os.path.dirname(__file__), 'resources', filename)
 
 
 def _canteen():