Converted TaylorF2 code to use PNCoefficients.

Also minor cleanup of PNCoefficients. Note that in the past, you could have higher spin order than PN order, but now those terms will be ignored.
Revert "Converted TaylorF2 code to use PNCoefficients."

This reverts commit 9475139f68b5786f0fe2eaf25863cf531c653611.
Use I0(snr^2 |autocorrelation|) for timing likelihood
Factor out signal model
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Faster, and less alias-y, method of computing autocorrelation
Implement new autocorrelation likelihood in Gaussian quadrature functions
Track API change in LALSimulation
Suppress underflow errors
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