Prevent data_paths that are unique to a component dataset in multi-assay upload

[gesinaphillips]

A prior spec allowed for a multi-assay upload where one or more component dataset metadata.tsvs could have unique data_path values, indicating that in addition to the multi-assay upload there were one or more standalone datasets of a component type. I.e., the following was permissible:

upload/
     parent-metadata.tsv
     component1-metadata.tsv
     component2-metadata.tsv
     path-referenced-by-all/
     path-referenced-just-by-component2/

This pattern is now invalid.

Note: examples updated means large number of files changed. All substantive changes are in upload.py.