DM-31489: Update StrayLightData to use FitsGenericFormatter with a deferred data set

[czwa]

This change makes the SubaruStrayLightDataFormatter a subclass of FitsGenericFormatter, and implements a readFits method to do so. The reads differ between the butler generations:

• Gen2: IsrTask.readIsrData calls SubaruStrayLightTask.readIsrData, which checks if the exposure needs correcting, and if so, calls the SubaruStrayLightData.readFits method. SubaruStrayLightData.run is called in the standard point in IsrTask.run.
• Gen3: A DeferredDatasetHandle is passed to IsrTask.run which calls SubaruStrayLightData.run at the standard point. The run method calls the same check method as in Gen2 (indicating this is run twice in Gen2), and if a correction is needed, converts the handle into the real object.

There are some slight naming clarifications (hdulist -> calib.ampData) and the docstrings in edited files have been converted to DM standards.