Refactor and reorder ISR steps to support writing pre-interpolated pixels

At DRP Team meeting April 3 2019, we discussed the steps in ISR that could be reordered in order to support storing pre-interpolated pixels (that have had ISR applied to them so that they can be meaningfully swapped in downstream processing).

The interpolation step before brighter-fatter is necessary. Therefore, we decided to reorder ISRTask as follows:

• First ISR Steps…
• Interpolate
• Do Brighter-Fatter Correction
• Put the pre-interpolated pixels back into the now BF-corrected raw.
• …Continue ISR…
• Saturated/NaN pixels and defects are masked only (no interpolation)
• Second to last step: write out pre-interpolated pixels
• Last step: Interpolate the union pixels masked for interpolation (sat/bad/nan)

Robert Lupton pointed out that it is more correct to interpolate the union of all regions that need to be interpolated rather than in separate stages for each of SAT, BAD, NaN. In addition to the extra steps of reinserting pre-interp pixels, this tickets also includes refactoring to separate the masking step and interpolation steps in maskAndInterpNan, maskAndInterpDefects, and maybe saturationIntepolation.