Resolve correct nirDark using IRPn ratio

Ramps taken with different dataPixel-to-IRP ratios have different per-read times, and so need different nirDarks. The correct way to do this is probably to add a registry dimension (per PIPE2D-1854), but that is pretty disruptive. Since we probably will not have values other than 1 and 4, simply add a nirDark_irp4 dataset type and:

• load the W_H4IRPN card as irp_ratio into the exposure record. That card has always been present.
• using irp_ratio, load the correct nirDark using a variant of the existing lookupBiasDark helper.