Dynamic spectral file selection

[nichollsh]

With my new tools for creating arbitrary spectral files, we now have more control over the number of bands used in the RT calculations. A significant optimisation to AGNI would be to dynamically switch the spectral file at run-time (at some time-cost). This would be done as follows:

1. Initialise T(p) with first guess and load semi-grey spectral file (3 bands)
2. Iterate rapidly for a while until we are somewhat closer to the solution.
3. Switch to medium-resolution spectral file (~200 bands) to obtain solution.
4. Switch to high-resolution spectral file (~4000 bands) to refine solution and generate nice spectra.

This comes with benefits to both performance and accuracy.