Prototype import "expansion" operation

[jamesaoverton]

We define an import table with a 'related' column, which we use to extract an OWL file with the explicitly listed terms and also their ancestors, children, etc.

When you look at the extracted OWL file, it's hard to tell why a specific term is included.

I would like an option to start with the import table and generate a new table of exactly which terms will be extracted -- call this the "explicit import" (bad name). The explicit import table will not have a 'related' column. Instead it should have a 'reason' (another bad name) column which says why this term will be extracted. Reasons should look like "ancestor of 'analyte assay', 'histology assay'", where 'analyte assay' and 'histology assay' are in the original import table.

I'm worried that there could be way too many reasons for terms near the root. Maybe there could be a limit, and then the reason would just be "ancestor of 5+ terms".

I think this will be useful, but I'm not 100% sure, so I would just like a prototype.

[beckyjackson]

The PR to resolve this issue is up-to-date and merge conflicts are resolved.