add more general checkpoint file name method

bfps/DNS.py

@@ -781,6 +781,10 @@ class DNS(_code):
         return os.path.join(
                     self.work_dir,
                     self.simname + '_checkpoint_0.h5')
+    def get_checkpoint_fname(self, checkpoint = 0):
+        return os.path.join(
+                    self.work_dir,
+                    self.simname + '_checkpoint_{0}.h5'.format(checkpoint))
     def generate_tracer_state(
             self,
             rseed = None,