make aligned spin in bns explicit

bilby/gw/prior_files/binary_neutron_stars.prior

@@ -8,8 +8,8 @@ mass_2 = Uniform(name='mass_2', minimum=1, maximum=2, unit='$M_{\\odot}$')
 # total_mass =  Uniform(name='total_mass', minimum=2, maximum=4, unit='$M_{\\odot}$')
 # mass_ratio =  Uniform(name='mass_ratio', minimum=0.5, maximum=1)
 # symmetric_mass_ratio =  Uniform(name='symmetric_mass_ratio', minimum=0.22, maximum=0.25)
-a_1 =  Uniform(name='a_1', minimum= -0.05, maximum= 0.05)
-a_2 =  Uniform(name='a_2', minimum= -0.05, maximum= 0.05)
+chi_1 =  bilby.gw.prior.AlignedSpin(a_prior=Uniform(0, 0.05), z_prior=Uniform(-1, 1), name='chi_1', latex_label='$\\chi_1$')
+chi_2 =  bilby.gw.prior.AlignedSpin(a_prior=Uniform(0, 0.05), z_prior=Uniform(-1, 1), name='chi_2', latex_label='$\\chi_2$')
 luminosity_distance =  bilby.gw.prior.UniformComovingVolume(name='luminosity_distance', minimum=10, maximum=500, unit='Mpc')
 dec =  Cosine(name='dec')
 ra =  Uniform(name='ra', minimum=0, maximum=2 * np.pi)