clean=True doesn't work with Pymultinest

If I set clean=True with multinest as the sampler, it uses the resume file unless I pass the resume=False which is the corresponding kwarg for pymultinest.run. I'm not really sure if the fix should go in the @kwarg_setter in pymultinest.py or in the _check_cached_result of base_sampler.py so I defer to someone who knows the structure better.