cpnest checkpointing

The default cpnest checkpointing kwargs are overwriting themselves. Setting resume=True doesn't actually start the checkpointing unless n_periodic_checkpoint is also set, and it defaults to None currently. I have changed the default value to 8000, which I picked somewhat arbitrarily based on the values for some of my previous dynesty runs. I have added a check in the verify_kwargs function that if resume==False and n_periodic_checkpoint is set, then change it to be None, so that the user doesn't need to know to set both. Let me know if there is a better way to handle this or to determine the right number of evaluations to use for the checkpointing.