structured_like can lead to supernumerous "grey" data sets when mapping over lists.

A tool that processes pairs and outputs pairs (which use structured_like and inherit_format from the input) produce supernumerous "grey" data sets.

Examples:

https://github.com/galaxyproject/galaxy/blob/dev/test/functional/tools/collection_paired_conditional_structured_like.xml
https://github.com/galaxyproject/galaxy/blob/dev/test/functional/tools/collection_paired_structured_like.xml


The number of grey data sets relates with the number of pairs in the input (for each output pair there are n-1 grey data sets for n pairs in the input). So it seems that the tool takes the wrong level.

Even if the tools might want to use `format_source` this should be fixed (and a test case for format_source for collections would be nice).

Furthermore I guess that the case with the conditional should have `structured_like="cond|input1"` instead of `structured_like="input1"`..?

Start with test/functional/tools/collection_paired_conditional_structured_like.xml and test/functional/tools/collection_paired_structured_like.xml, then trace the structured_like and inherit_format handling for mapped lists. Reproduce the examples and verify that output pairs no longer create supernumerous grey datasets; consider the conditional structured_like form and a collection format_source test described in the issue.

Written by the indexing model from the issue text.