Fix normalisation of fibers in flats

We need to be able to calibrate all fibers on a CCD by calibrating just one, so the fiber flats need to preserve the fiber-to-fiber throughput variations, whereas currently they don't.

In the process, remove DetectorMap._throughput: throughput variations will be taken care of in the flat-field, and that also allows us to correct throughput as a function of wavelength rather than a single value per fiber.