Permit Readnoise as number

[oczoske]

Currently, FPA_READOUT_MEDIAN is passed to nghxrg to create a noise image. This is only applicable to the HAWAII detectors. It ought to be possible to bypass nghxrg and use FPA_READOUT_MEDIAN as the average readnoise to be realised by a Gaussian process.

[astroleo]

The dark current should also be added like this, although the read noise is more important for thermal IR applications.