ENH: Add support for sampler plugins

.gitlab-ci.yml

bilby/core/sampler/__init__.py

bilby/core/utils/__init__.py

bilby/core/utils/entry_points.py

docs/index.txt

docs/plugins.txt

requirements.txt

setup.py

test/core/sampler/base_sampler_test.py

test/core/sampler/cpnest_test.py

test/core/sampler/dnest4_test.py

test/core/sampler/dynamic_dynesty_test.py

test/core/sampler/dynesty_test.py

test/core/sampler/emcee_test.py

test/core/sampler/general_sampler_tests.py

test/core/sampler/kombine_test.py

test/core/sampler/nessai_test.py

test/core/sampler/nestle_test.py

test/core/sampler/ptemcee_test.py

test/core/sampler/pymc_test.py