feature request: if flattening is turned on, output flattened target spectrum

[rcthomas]

Sourced to Kate. It would be nice if there was a way to have the target spectrum output with the flattening correction applied to it. This is easy to do in synapps, since synapps knows target spectrum and could output the flattened version by just correcting it. If one wants to take the flattening correction from a syn++ input file and apply it, syn++ would need to know the target spectrum. An option "--target" could be added which would not necessarily duplicate or collide with "--wl-from" since the argument of the latter option needn't necessarily be the target spectrum.