Merge branch 'improve-prior-reading' into 'master'

bilby/core/prior.py

bilby/gw/prior_files/GW150914.prior

bilby/gw/prior_files/aligned_spin_binary_black_holes.prior

bilby/gw/prior_files/binary_black_holes.prior

bilby/gw/prior_files/binary_neutron_stars.prior

bilby/gw/prior_files/precessing_binary_neutron_stars.prior

test/gw_prior_test.py

test/prior_files/binary_black_holes.prior

test/prior_files/binary_neutron_stars.prior

test/prior_test.py