Keywords describing sigma

New keywords that completely describe how to calculate SIGMA. These will be added to SPICE level 3 files.

SIGMADAT  = 'sqrt(RADCAL*GAIN*NOISEFAC^2*(DATA>0) + DARKSUBF*NBIN*(READNOIS^2 + DARKCURR*XPOSURE) / RADCAL'

GAIN     = xx.xx   / [DN/photon] Camera conversion gain
NOISEFAC = xx.xx   / Intensifier noise factor
READNOIS = xx.xx   / [DN] Readout noise
DARKCURR = xx.xx   / [DN/s] Dark current
DARKSUBF = 1 or 2  / Dark subtraction noise factor
RADCAL   = xx.xx   / [DN/(W m-2 sr-1 nm-1)] Radiometric calibration factor

DARKSUBF = 2 except if PRREFn = "combined_dark_xxx" (where n is from PRSTEPn = "DARK-SUBTRACTION").

RADCAL is actually a variable keword, and we will use these variable values instead of the average given in the header keyword. We will also save these variable values into the Level 3 file.

Data < 0

Pixels with value < 0 will be set to 0 to calculate sigma.

Hot/CR pixels

Set CR(?) pixels and hot pixels to NAN. There is / will be a Pixellist in an HDU in level 2 OR in level 3.