user option to force source shapes during blind source finding

[PaulHancock]

Add a user option like --minpsf that will set the minimum source size to be equal to the psf. Potentially also allow --psfonly that will force the shape of every source to be equal to the psf.

Pro: Make some people happy who don't like seeing sources being fit with ellipses that are un-physical Con: This will result in unknown a/b/pa errors for many sources.