correct for the focal plane variation of the flux calibration vector

After applying the fix for PIPD2D-1361, we clearly observe the spatial variation of the flux calibration vector. The variation likely has two components; the global pattern due to the global offset, rotation, and scale difference, and fiber-to-fiber offset. We want to correct for the former with, e.g., low-order polinomials. As the seeing is a function of wavelength, a fiber offset introduces a wavelength-dependent flux loss. We thus need to allow the flux calibration vector to vary both spatially and along the wavelength. The correction can be estimated by comparing the PS1 magnitudes and synthetic PS1 magnitudes of each FLUXSTD.