Add failing unit-tests illustrating forced-fit issues.
Add failing unit-tests illustrating forced-fit issues.

Two problems here:
 - We need to be more careful when avoiding forced fits that are not
  possible due to the source largely being obscured by NaN pixels.
  (Typically these are used around the edges of a mosaic image.)
 - The unit-test to ensure a fit *outside* of an image was rejected,
  was passing erroneously. The old version generated a test-location
  by performing arithmetic on sky-coordinates, but (I think) did
  not account for the fact that the test-data is a polar image,
  and so the RA wraps around! The new version simply picks a pixel
  location that will *always* be outside the image, and converts that
  back to a sky-position for feeding to the forced-fit routines.

transientskp/tkp,transientskp/tkp