Download spectral files as required

[nichollsh]

Some spectral files are automatically downloaded and stored in the $FWL_DATA directory. However, this does not necessarily mean that the spectral file that you want is downloaded. It would be ideal if PROTEUS automatically downloaded the appropriate file from OSF if it cannot find it.

This is connected with #74, since we could have the configuration file specify a required spectral file "group" (e.g. Frostflow) and "resolution" (e.g. 256) rather than a file path which may or may not exist.

[nichollsh]

This should be made a lot easier once the new configuration file format is implemented. The spectral files will be specified by:

• spectral_group with a codename referring to the included opacities/data (e.g. Honeyside)
• spectral_bands with the number of bands (e.g. 256, but stored as a string)

See #74