Michael Puerrer authored Apr 15, 2019 and Sebastian Khan committed Apr 15, 2019

Frequency domain surrogate of the time-domain SEOBNRv4T
effective-one-body waveform model for spin-aligned binary neutron
stars including overall tidal deformability using universal relations.

At low frequencies the approximant returns spline interpolated TaylorF2
on sparse grids. At higher frequencies the waveform consists of
TaylorF2 + surrogate amplitude and phase corrections to approximate SEOBNRv4T
(previously called TEOBv4). TaylorF2 is evaluate once on a sparse grid.

The HDF5 data file contains the GPR surrogate training data and
sparse grids tailored to linear or cubic spline interpolation.

Here we subtract off the 3PN spin-spin term that is not included in SEOBNRv4T.
The quadrupole monopole parameters are set using universal relations.

Add coordinate transformation between lambda and xi.
Implement pn_ss3 using TF2 functions.