MRG: modify annotate for flexibility

[bluegenes]

tax annotate does not need to require the gather columns that are required for tax metagenome and tax genome.
These got especially strict after the recent taxonomy refactor.

This PR modifies tax annotate so that it is just looking for a column it can match to taxonomy assignments. This will allow annotation of search, prefetch files and any other file with a good column name.

(new) Acceptable column names, in the order annotate will look for them:

• name (in search, gather output)
• match_name (in prefetch output)
• ident
• accession

Are there any others we should add? We could also let the user specify a column name via cli input, but I don't see the need to enable it unless requested?

Closes #2472

[ctb]

nice and clean! LGTM!