* Add free_mem method in CatalogBase.py. Thus class that inhrits this base class can then implement this function to free used memory.

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