Optimal angle, refactored gas optics
Refactored gas optics increases efficiency and reduces memory use. Optimal angle data is added to LW coefficient files; single-angle no-scattering calculations making use of this data are more accurate than using the standard diffusivity angle. See lw_clear_sky_optangle() in tests/clear_sky_regression.F90 for an examples. Other bug fixes (e.g. to multi-angle no-scattering calculations using default kernels), increased consistency between GPU and CPU kernels; small misc. improvements.