Bias subtraction

In most cases there is no bias to subtract. Recipes attempt to remove a bias frame only if the data have variance information and were taken using a non-ND mode (i.e. where the bias has not already been subtracted in the instrument data system), whereupon a bias frame, if available, is subtracted; if, however, there is no bias calibration the recipe issues a warning issued that the computed variances may be wrong. This step occurs between the creation of the readnoise variance and adding the Poisson variance.
[_REMOVE_BIAS_]