Store the taxon_str on the accession record

[brettatoms]

It would be way faster to store this on the record rather than query it every time the accession record is looked up. We could hook into an SQLAlchemy after_save event on the Taxon to update the taxon_str on its children accession when it is saved.

If we have taxon_str cached on the accession then we could return it in json() with depth=1