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.