Fits the PSF to a radial distribution produced by IRADIAL and displays
the result. The fit is a weighted least-squares - the reduced chi-
squared and equivalent normal z is output. The latter is not useful
unless the number of radial bins is > 
10. The first time IPSF is
invoked with a particular image, the user is required to choose the
psf to use. The choice will be 'instrument-specific' or 'analytical'.
Once chosen, this will be used by default with subsequent invocations.
The value and error of the background may also be specified.
There is an option to choose a new PSF.