Apply proper distortion function for AG to make pfsConfig

To calculate predicted position of the guide star candidates when pfsDesign (and pfsConfig) is created, "sky_pfi" is called now. However, this function has been optimized for fiber positions through the previous engineering runs (and also distortion coefficient is different), The predicted position is different by order of 100um for the same sky target now (as a result of optimization). We should use dedicated function for AG.