Overhaul of Result() implementation

CHANGELOG.md

bilby/core/prior.py

bilby/core/result.py

bilby/core/sampler/__init__.py

bilby/core/sampler/base_sampler.py

bilby/core/sampler/dynesty.py

bilby/core/sampler/pymc3.py

test/result_test.py

test/sampler_test.py