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

# Conflicts:
#   observation_sim/mock_objects/Galaxy.py
#   observation_sim/mock_objects/MockObject.py
#   observation_sim/psf/PSFInterpSLS.py