Discuss strategy to persist pfsFiberTrace

To persist the new `FiberTraces` after the recent changes to the design of the `FiberTrace` class we need to change the datamodel and persist the `MaskedImage` for each `FiberTrace`. There are currently 2 proposals how to do that. One would be to cat all `MaskedImage` s into one single `MaskedImage` and also add an additional data array for the unpacking. This could possibly be achieved by saving the `MaskedImage` first using the `MaskedImage.writeFits` method and then add all additional data to that fits file using `pyfits`. The second proposal would be to find a way to store multiple `MaskedImage` s in one fits file - one `MaskedImage` per `FiberTrace`. This appears to be rather complicated as the `MaskedImage` class was not designed to put multiple arrays into one file. Changes to the `MaskedImage` class might be required