Improve how sampling seed is handled

bilby/bilby_mcmc/sampler.py

bilby/core/sampler/base_sampler.py

bilby/core/sampler/cpnest.py

bilby/core/sampler/dnest4.py

bilby/core/sampler/dynesty.py

bilby/core/sampler/emcee.py

bilby/core/sampler/nessai.py

bilby/core/sampler/nestle.py

bilby/core/sampler/polychord.py

bilby/core/sampler/ptemcee.py

bilby/core/sampler/ptmcmc.py

bilby/core/sampler/pymc3.py

bilby/core/sampler/pymultinest.py

bilby/core/sampler/ultranest.py

test/core/sampler/base_sampler_test.py

test/core/sampler/nessai_test.py