Updated CellImg classes for extensibility

* Factored out AbstractCellImg
* Factored out AbstractCellImgFactory
* Updated implenetations to extend these abstract classes
* Refactored supporting classes (Cursors, RandomAccess, IterationOrder)
  to use the Abstract layer.
formatted to imglib2 style
renamed AbstractCellImg.copy() to copyDataTo() to avoid confusion with Img.copy()
Merge branch 'master' into scifiocells