Combine DCTs (DSTs) and connection coefficients for faster (alternative) synthesis and analysis

[MikaelSlevinsky]

Using an O(n² log n) instead of O(n²) precomputation, the colatitudinal DCTs and DSTs can be applied to the Legendre to Chebyshev connection coefficients to reduce the overall spherical harmonic synthesis and analysis to execute_sph_hi2lo followed by a sequence of cblas_dgemm and one longitudinal FFTW_HC2R.

Contrast this with the current four-step approach of execute_sph_hi2lo followed by a sequence of cblas_dtrmm and a sequence of FFTW_REDFT01 and FFTW_RODFT01, and finally the longitudinal FFTW_HC2R.