remove the read noise terms (first add to then minus from base image) from possion noise generation.

1. add project_cycle, run_counter to config file. Modified obsid to reflect this change. Applied the same obsid to ".cat" and ".log" files.
2. limiting mags are now based on the field types (WIDE or DEEP) which can be specified in pointing list.
3. Stars are now drawn directly as PSF.
4. Reorgnaized lists of filter types in Instrument._util.

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