Started implementation of missing flux calculation for SBSpergel
Merge branch '#616' of https://github.com/GalSim-developers/GalSim into #616

# By jmeyers314
# Via jmeyers314
* '#616' of https://github.com/GalSim-developers/GalSim:
  start implementing Spergel HLR (#616)
  Some Spergel test images and a unit test (#616)

Conflicts:
	src/SBSpergel.cpp