Remove at least one of the spline implementations in drp_stella

I discovered that there are two separate spline interpretations in drp_stella, and we should remove at least one of them. There are unnecessary copies of arrays in FiberTraces.cc that should be removed when we clean up the interpolation.

There is functional spline interpolation in {lsst::afw::math::Interpolate}} , but it only supports std::vector<double>. If it is appropriate to use doubles wherever we need to interpolate in drp_stella we should definitely switch to use the afw implementation.