Do not fiddle with slit offsets by default in reduceArc

The default value of config.reduceExposure.doOffsetDetectorMap for reduceArc is True; please change this to False.

The slit offsets are a feature of the hardware, resulting from exactly how we epoxy the fibres into the slit at LNA. We should therefore only change them after very careful analysis, and certainly not every time that we update the arc solution.

In particular, allowing the offsets to float causes running reduceArc on SuNSS arc data to miss real fibres moderately often. By turning this off, all 195 sky frames that I looked ran successfully in both b and r.