An efficient spectral interpolation routine for the TwoPunctures code

TwoPunctures is perhaps the most widely-adopted code for generating binary black hole "puncture" initial data and interpolating these (spectral) data onto evolution grids. In typical usage, the bulk of this code's run time is spent in its spectral interpolation routine. We announce a new publicly-available spectral interpolation routine that improves the performance of the original interpolation routine by a factor of ~100, yielding results consistent with the original spectral interpolation routine to roundoff precision. This note serves as a guide for installing this routine both in the original standalone TwoPunctures code and the Einstein Toolkit supported version of this code.