Support for raw exposures with W_PFDSGN not set

This issue is raised following a discussion on slack channel #drp-2d on 2019-04-09 and today.

The FITS keyword W_PFDSGN has been set in raw LAM exposures since approx 2010-02-18. The current 2D DRP pipeline currently supports data with this keyword, but not well in scenarios without this keyword. There are a number of developers needing to process LAM exposures before and after this change, so the 2D DRP pipeline needs to be updated to support both scenarios.

So the scenario where W_PFSDSGN is not set needs to be supported. It needs to be agreed under this scenario what fibers are assumed to be illuminated (all? none?). If it is assumed that no fibers are illuminated, then PfiDesign needs to be updated, as it currently will not persist an instance if itself if the fiberId is an empty list.