Subtract off the start_obj_num offset when writing checkpoint files so that they can be used individually or in other combinations with galsim multiprocessing.
Add a checkpoint_sampling_factor config parameter to reduce the number of checkpoints written to be a fraction of the number of stamp batches.
start_obj_numoffset when writing checkpoint files so that they can be used individually or in other combinations with galsim multiprocessing.checkpoint_sampling_factorconfig parameter to reduce the number of checkpoints written to be a fraction of the number of stamp batches.