SpectrumSet.makeImage assumes that SpectrumSet and FiberTraceSet are aligned

There's a bug in the book-keeping in SpectrumSet.makeImage:

assert len(self) == len(self), "Number of spectra and fiberTraces don't match" 

(self appears twice) which means that the code accepts being passed a set of fiberTraces that doesn't match the spectra; this happens when there are broken fibres (think SuNSS); (PIPE2D-819).

Furthermore, the code assumes that the order of fibres in SpectrumSet and FiberTraceSet are the same; is that guaranteed? It might be safer to switch to

fiberIds = list(self.getAllFiberIds())
for ft in fiberTraces:
    spec = self[fiberIds.index(ft.fiberId)]
    fiberImage = ft.constructImage(spec)
    image[fiberImage.getBBox(), afwImage.PARENT] += fiberImage