Smart binning and spline interpolation

This release brings a number of critical performance improvements. First, the mean galaxy occupation number in each halo bin is now properly integrated over the bin using Gaussian quadrature instead of taking the number at the bin midpoint. This allows TabCorr to get high accuracy with a smaller number of bins. In turn, this improves performance for both tabulation and prediction and results in smaller file sizes. Also, the interpolation framework now supports spline interpolation which is much more accurate than the older linear barycentric interpolation. Thus, for example, one now needs less tabulated correlation functions to interpolate over phase-space parameters. Finally, multithreading now makes use of shared memory.