Resolve "Follow-up from "Adding dnest4 Sampler""

bilby/core/sampler/__init__.py

bilby/core/sampler/base_sampler.py

bilby/core/sampler/dnest4.py

bilby/core/sampler/pymultinest.py

bilby/core/sampler/ultranest.py

test/core/sampler/dnest4_test.py

test/core/sampler/sampler_run_test.py