Improved narrow-band and broad-band shortwave spectral files including photolysis

A number of improvements are required for the new spectral files:
* Improved treatment of the solar spectrum
* Optimisations to allow 2000-band spectral files
* Functionality to allow direct flux only calculations for specific k-terms
* Application of Rayleigh scattering coefficients per k-term rather than per band
* A new weighted UV-index diagnostic (calculated over sub-bands) for assessing accuracy of the broadband file