this is an important change. I have changed the way in which the psf elliptical contour parameters are defined and fitted. SXX,SYY -> 1/SXX,1/SYY (ie, they are now ~ sigma, not ~1/sigma). SXY is now fitted with the form SXY/(1/SXX2 + 1/SYY2)2
also added some additional tests and checks