Resolve "add neutron star merger to examples"

I added the lal_binary_neutron_star model with an example file binary_neutron_star_example.py . I had to include an import lal in the file source.py to add tidal deformabilities.

For the bns approximants TaylorF2 and IMRPhenomD_NRTdidal, the parameters tilt_1, phi_12, tilt_2, phi_jl must be set to zero, since these can only handle aligned spins.