Add missing natoms definition at failback when heroistic reading of trajectory...

Add missing natoms definition at failback when heroistic reading of trajectory without topology info

common/python/nomadcore/md_data_access/MDDataAccess.py

@@ -957,6 +957,7 @@ class MDDataAccess(object):
             }
 
     def initialize(self):
+        self.natoms = None
         self.topology = None # Main storage for topology data
         self.trajectory = None # Main storage for trajectory data
         self.forcefield = None # Main storage for force field parameters