1. bright objects were missing in some bands.
2. saturation cut magnitudes were different for the same objects in different bands

* Now use exp_time for single exposure for saturation mag calculation. Use for depth exp_time for limiting mag calculation.

This reverts commit 638b95d7

This reverts merge request !28

2. change version number to 3.0.0rc

issue17:fix led model memory bug; issue19:header time is not utc bug; issue18:header data-obs\darktime\exposure time fix by shaoli's chart;add new star catalog:C6_50sqdeg_ns.py

* Changed the name of output folder of pointing to corresponding pointing.obs_id.
* fixed the order .cat hdr updating and the creation of .cat file.
* Changed the naming of shutter and flat reference image: chip IDs now all in 2 digits.
* Updated the __init__ method of ChipOutput
* Removed Config.py from ObservationSim.Config
* Cleaned codes in ObservationSim.py, ChipOutput

remove pointing_dir, data_dir in overall_config