fix for broken pixelClassification tutorial script

- generalize transform_flatfields a bit (check for FlatField instead of
  SingleBandedImage; use makeFlatFieldSequence instead ImageSequenceImpl
  constructor to make time sequences)
- Add keepMetadata decorator to more arithmetic formulas.  This is
  temporary because these decorators should be unnecessary upon
  completion of inq 1720
- makeFlatFieldSequence handles the timestamp associated with data
  objects resulting from getADDEImage (only handled loadFile stuff
  previously)

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