1. Update the field distortion models for FGS detectors (with new wcs definition). 2. Update README.md

1. remove *3600 for rv in Astrometry_util.py. 2. fix a typo in an if statement for add_bias option in Chip.py

2. Hotfix: fixed a bug when get the bandpasses for spectroscopic chips in Filter.py

2. swith to use "interFlux" in CatalogBase.py,
so now the implementation for number of photons been
shot in Photometric and Spectroscopic are the same
3. Minor config option fix in run_sim.py

fix discrepancy by applying saturation cut in each band, add limiting magnitude calculator, fix randomization problem in SED assignment