Study difference in guide star position for pfsConfig and ag's predicted position

The predicted position of the guide stars at the time of convergence (i.e., pfs_config.to_be_observed_at) is different from position predicted by AG (converted from agc_pfi_nominal_x_mm and y), even the latest CCD position (INSTRM-2654) and a set of dedicated routines for AG (as a part of INSTRM-2658) is used. Look int these difference.