One more bug fix for iterate option in BACKGROUNDS

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This commit contains the updates for version 0.0.2.1.

The main changes are as follows:

1. If sampling redshfits from USERDISTs or DISTRIBUTIONs caused unphysical geometries (e.g. PLANE_2 having a REDSHIFT less than PLANE_1), the simulated system is deleted from the images and metadata.
2. The BACKGROUNDS section of the configuration file now accepts an ITERATE key for which the value is unused (set it to None). If ITERATE mode is used, the images are used in the simulations iteratively which ensures that all images get used.

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