allow precessing BNS

This MR allows the lal_binary_neutron_star source model to be used with precessing spins.

It doesn't put this in by default and doesn't change the API.

I did change the default BNS waveform approximant to be IMRPhenomPv2_NRTidal rather than TaylorF2. The latter is inspiral only, whereas the former includes the standard lalsimulation treatment of tides through the merger. It also works with precessing spins.

bilby/gw/prior_files/precessing_binary_neutron_stars.prior

+# These are the default priors we use for BNS systems.
+# Note that you may wish to use more specific mass and distance parameters.
+# These commands are all known to bilby.gw.prior.
+# Lines beginning "#" are ignored.
+mass_1 = Uniform(name='mass_1', minimum=1, maximum=2, unit='$M_{\\odot}$', boundary=None)
+mass_2 = Uniform(name='mass_2', minimum=1, maximum=2, unit='$M_{\\odot}$', boundary=None)
+mass_ratio = Constraint(name='mass_ratio', minimum=0.125, maximum=1)
+# chirp_mass = Uniform(name='chirp_mass', minimum=0.87, maximum=1.74, unit='$M_{\\odot}$', boundary=None)
+# total_mass = Uniform(name='total_mass', minimum=2, maximum=4, unit='$M_{\\odot}$', boundary=None)
+# mass_ratio = Uniform(name='mass_ratio', minimum=0.5, maximum=1, boundary=None)
+# symmetric_mass_ratio = Uniform(name='symmetric_mass_ratio', minimum=0.22, maximum=0.25, boundary=None)
+a_1 = Uniform(name='a_1', minimum=0, maximum=0.05, boundary='reflective')
+a_2 = Uniform(name='a_2', minimum=0, maximum=0.05, boundary='reflective')
+tilt_1 = Sine(name='tilt_1', boundary='reflective')
+tilt_2 = Sine(name='tilt_2', boundary='reflective')
+# cos_tilt_1 = Uniform(name='cos_tilt_1', minimum=-1, maximum=1, boundary=None)
+# cos_tilt_2 = Uniform(name='cos_tilt_2', minimum=-1, maximum=1, boundary=None)
+phi_12 = Uniform(name='phi_12', minimum=0, maximum=2 * np.pi, boundary='periodic')
+phi_jl = Uniform(name='phi_jl', minimum=0, maximum=2 * np.pi, boundary='periodic')
+luminosity_distance = bilby.gw.prior.UniformSourceFrame(name='luminosity_distance', minimum=10, maximum=500, unit='Mpc', boundary=None)
+dec = Cosine(name='dec', boundary='reflective')
+ra = Uniform(name='ra', minimum=0, maximum=2 * np.pi, boundary='periodic')
+theta_jn = Sine(name='theta_jn', boundary='reflective')
+# cos_theta_jn = Uniform(name='cos_theta_jn', minimum=-1, maximum=1, boundary=None)
+psi = Uniform(name='psi', minimum=0, maximum=np.pi, boundary='periodic')
+phase = Uniform(name='phase', minimum=0, maximum=2 * np.pi, boundary='periodic')
+lambda_1 = Uniform(name='lambda_1', minimum=0, maximum=3000, boundary=None)
+lambda_2 = Uniform(name='lambda_2', minimum=0, maximum=3000, boundary=None)
+# lambda_tilde = Uniform(name='lambda_tilde', minimum=0, maximum=5000, boundary=None)
+# delta_lambda = Uniform(name='delta_lambda', minimum=-5000, maximum=5000, boundary=None)