not tested --- tweak checkpointing

make sure code stops at multiples of niter_todo, or when
the file "stop_<simname>" is found.
i.e. if i stop it at iteration 3, and niter_todo is 16, it should next
stop at 16, not at 19.

bfps/_code.py

@@ -32,6 +32,7 @@ import argparse
 import h5py
 from datetime import datetime
 import math
+import warnings
 
 import bfps
 from ._base import _base

bfps/_fluid_base.py

@@ -264,7 +264,7 @@ class _fluid_particle_base(_code):
                                       '<< time_difference/nprocs << " seconds" << std::endl;\n' +
                                   'time0 = time1;\n')
         if not postprocess_mode:
-            self.main       += 'for (int max_iter = iteration+niter_todo; iteration < max_iter; iteration++)\n'
+            self.main       += 'for (int max_iter = iteration+niter_todo-iteration%niter_todo; iteration < max_iter; iteration++)\n'
             self.main       += '{\n'
 
             self.main       += """