Converts a optical transfer function specified over two-dimensional  
spatial frequency in cycles per degree to a point spread function  
specified over positions in minutes.  For human vision, these are each  
natural units.  
  
The input spatial frequencies should be specified in matlab's grid  
matrix format and sf for x and y should be specified over the same  
spatialfrequency extent and with the same number of evenly spaced  
samples. Spatial frequency (0,0) should be at location floor(n/2)+1 in  
each dimension.  The PSF is returned with position (0,0) at location  
floor(n/2)+1 in each dimension.  This is the form we mostly want to  
look at and use.    
  
The OTF is assummed to have the DC term in the center poistion,  
floor(n/2)+1 of the passed matrix.  If you have your hands on an OTF  
with the DC term in the upper left position (1,1), apply fftshift to  
it before passing to this routine.  The DC in upper left is the Matlab  
native format for applying the ifft, and is also the format stored by  
isetbio in its optics structure.  
  
Positions are returned using the same conventions.  
  
No normalization is performed.  The psf should be real, and we  
complain (throw an error) if it is not, to reasonable numerial  
precision. If it seems OK, we make sure it is real.  
  
We also make sure that the returned psf is all postive and sums to   
1.  In some cases, we found that there were small negative values  
and after setting these to zero renormalization was needed.  
  
We wrote this rather than simply relying on Matlab otf2psf/psf2otf  
because we want a routine that deals with the conversion of spatial  
frequency to spatial support.  
  
If you pass the both sf args as empty, both position grids are  
returned as empty and just the conversion on the OTF is performed.  
  
[PsychOpticsTest](PsychOpticsTest) shows that this works very well when we go back and  
forth for diffraction limited OTF/PSF.  But not exactly exactly  
perfectly.  A signal processing maven might be able to track down  
whether this is just a numerical thing or whether some is some small  
error, for example in how position is converted to sf or back again in  
the [PsfToOtf](PsfToOtf).