Merge branch 'improve_cosmology_use' into 'master'

CHANGELOG.md

bilby/core/prior.py

bilby/gw/__init__.py

bilby/gw/conversion.py

bilby/gw/cosmology.py

bilby/gw/prior.py

examples/injection_examples/fast_tutorial.py

test/conversion_test.py

test/cosmology_test.py

test/gw_prior_test.py

test/prior_test.py