Accept more than one reference lamps

[wschoenell]

When doing automatic wavelength calibration, I know I can choose a different folder for reference lamps. For each configuration, redspec expects a single reference file. When I add another reference file for a given configuration, the program gives me an error message saying that does not know which one to use.

I believe redspec should be able to decide which reference file to use based on the closest date of the data being reduced.