Add option to use the fallback value at the edge of the image

Also unpack variables to allow us to implement this (but don't do so yet)
Improved comment on defectType
Merge remote-tracking branch 'mitaka/tickets/DM-670' into mitaka_master
Adapt to new mask bit NO_DATA replacing EDGE (DM-669)

NO_DATA means a warp/coadd pixel doesn't map to a CCD.
EDGE means a pixel is near the edge and couldn't be searched
for sources.
Merge branch 'tickets/DM-669'
Add CoaddBoundedField class to coadd aperture corrections
Merge branch 'tickets/DM-798'
New subtask for measuring aperture corrections
Fixes to aperture correction measurement
Remove parts of CorrectFluxes we plan to replace
Removed ScaledFlux inheritance from GaussianFlux, PsfFlux
Remove ScaledFlux algorithm; obsoleted by apCorrRegistry
Update CorrectFluxes algorithm to use new aperture correction map
Record per-source, per-algorithm aperture corrections by default
Support running measurement over a range in algorithm priority
Remove non-functional apCorrRegistry mechanism
New C++ registry for aperture-corrected fields

Now, instead of trying to use the registry to set the defaults of a configurable
list of to-be-corrected fluxes (which didn't work), we'll use the full registry, minus
any algorithms in a to-be-ignored list.
Minor fixes from code review of DM-191
Warn and continue when we don't have enough sources to compute aperture corrections
Merge branch 'u/jbosch/DM-191'