Spartacus shortwave optimizations + avoiding double prec. in two-stream kernels

[peterukk]

Hello,

Here are the performance optimizations I discussed. Most of them target the expm computations in spartacus_sw. In my tests, the solver was about 45% faster (single core on an Intel laptop, ifort -O3).

Also included in this pull request is a fix to the two-stream kernels which removes the need for any double precision variables/computations by instead adjusting the minimum "k" parameter based on working precision. This change should be tested some more - I only tested the McICA and Spartacus shortwave and longwave solvers and found that net upwelling and downwelling fluxes for the test IFS profiles changed by less than 0.1 W/m2. The benefit is increased performance for single precision computations; 20-25% faster mcica_sw and mcica_lw in my tests. If you want, I can also scrap this and make a pull request for only spartacus_sw.

[FussyDuck]

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[rjhogan]

Many thanks for this! Could I request the following changes please:

• Rather than a separate routine calc_reflectance_transmittance_sw_opt, please provide the alternative definitions of calc_reflectance_transmittance_sw within #ifdef USE_RTE_REFTRANS_SW #else #endif. This is because this routine is used in many more places than you have shown, and to easily switch between the two at compile time in all places.
• Modification dates have a valid date (not -xx) - the exact date doesn't matter
• No single-letter variables (so k becomes k_exponent), much easier for searching through a file
• Lower case for variables (so RT_term becomes reftrans_factor) - see radiation/CONVENTIONS
• Camel case for parameters so k_min becomes KMin - see radiation/CONVENTIONS
• IFS conventions also require that the continuation character (&) is provided both at the end of a line and at the beginning of the continuation line Thanks!