Cleanup LayeredImage PSF usage

[jeremykubica]

This is a cleanup PR aimed at simplifying how LayeredImage uses the PSF:

• Move add_object from LayeredImage to fake_data_creator.py. This function is only used in testing to create fake data and is a better fit to fake_data_creator.py. This also removes one of LayeredImage's use of PSFs.
• Add no-op PSF constructor for testing. This can also be used to make PSFs effectively optional in LayeredImage by passing in a no-op.
• Add a convolve function to LayeredImage that takes an external PSF (with the goal of making PSFs in LayeredImage optional).
• Remove psf_sq as a class variable for LayeredImage. Recomputing it is not expensive compared to the subsequent convolve operations.