Error bits from one band can affect calculations in another sometimes - which leads to lost source information.

[jimfinnis]

Error bits limit the pixels used for calculating spectra etc.

Suppose we set an error bit on ALL bands if ANY band has a dodgy pixel.

Calculations on any band which involves those bad pixels will be using information generated from all bands (i.e. image data from one band but error data from all bands), but this will not be apparent from the sources.

Related to #68

Will add comment to dqmod.