Improve treatment of solar spectrum for correlated-k

[mo-jmanners]

A number of improvements are required for the treatment of the solar spectrum when generating spectral files:

• When the solar spectrum is of lower resolution than the spectral file sub-bands the spectral weightings are poorly interpolated.
• The spectral variability file needs to allow solar variations at lower resolution than the spectral file sub-bands.
• The solar weighting for correlated-k terms can be improved by supplying appropriate attenuated spectra for each pressure level.