Add initial time as hdf5 attribute in callbacks

- This is easier to directly load than the units 'time since initial time'
- still keep the units

thetis/callback.py

@@ -209,6 +209,7 @@ def __init__(self, solver_obj, array_dim=1, attrs=None,
         if init_date is not None and include_time:
             time_units = 'seconds since ' + init_date.isoformat()
             self.var_attrs['time'] = {'units': time_units}
+            self.attrs['simulation_initial_date'] = init_date.isoformat()
 
     def set_write_mode(self, mode):
         """