reduceArcs should include all the fibres in the output DetectorMap

When I fit a DifferentialDetectorMap in reduceArc, it only includes the detected fibres, not all of the ones in the base DetectorMap.

Paul points out that this is because if a fiber doesn't have any measured lines, we can't measure a slit offset for it, and therefore we don't really know where it is. However, we should assume that slit offsets are measured once and left fixed thereafter as they are a property of the epoxy in the slit and should not change with time. This means that something like reduceArc which updates the DetectorMap shouldn't touch them unless specially requested to do so.

We do still have to measure them initially. For broken fibres the values don't matter (in the case of SuNSS the fibres are not broken in DCB data, although fiberId 3 is very faint). For all other fibres when the spectrograph is first delivered we'll take lots of spot data, quartz, arcs, ... and turn on all the "slit offset" options to measure the soon-to-be-fixed values.