Implement I0(snr^2 |autocorrelation|) likelihood
Use Gauss-Legendre quadrature over cos(i) and distance

Also add unit tests.
Tune thinning and number of samples per bin
Add option to postprocess MCMC samples with KDE
Match polarization sign in signal_amplitude_model & realize_coincs

I have checked that this is the sign convention that is used by
gstlal_inspiral.

This patch also adds a unit test for signal_amplitude_model.
Factor out awkward type punning
Conjugate nominal SNR to match convention in paper

Likelihood should depend on zhat * conj(z)
Adapt to renamed SWIG bindings
Use crude adaptive mesh refinement in place of TOA-only localization
Disable GSL 1.15-only code if GSL is too old
Fudge factor for excess estimation error in gstlal_inspiral
Remove TOA-only localization

One less thing to review!
Conditionally disable some more GSL >= 1.15 code