Perform at least one SDC iteration (Parallel-in-Time#509)

* Perform at least one SDC iteration

* Can use only prediction if predicting exotically

* Reverted last commit

Author: brownbaerchen

pySDC/implementations/convergence_controller_classes/check_convergence.py

@@ -73,7 +73,7 @@ def check_convergence(S, self=None):
 
         # get residual and check against prescribed tolerance (plus check number of iterations)
         iter_converged = S.status.iter >= S.params.maxiter
-        res_converged = L.status.residual <= L.params.restol
+        res_converged = L.status.residual <= L.params.restol and (S.status.iter > 0 or L.status.sweep > 0)
         e_tol_converged = (
             L.status.increment < L.params.e_tol if (L.params.get('e_tol') and L.status.get('increment')) else False
         )