Add arbitrary weighting of each spectra

[biprateep]

No description provided.

[dirkscholte]

Does this mean an arbitrary weight per spectrum or an arbitrary grid of weights per spectrum and per wl bin? I have the first of the two in a branch in my fork but not the second as of yet as it's a bit more fiddly with the co-addition of the different cameras etc. (I've also included an ivar-weighted-mean in this fork.)