Add definition for "strain"

[DanBerrios]

Taxon doesn't generally have definitions, and that's appropriate for most of the concepts in the database. However, the ranks could at least be defined, no? We would like to point to some ontology-based concept definition for strain, and the one in OBI is limited to selectively maintained organisms.....

Britannica says: "Among domestic animals, a true-breeding, genetically pure line is usually called a strain."

If this issue has been raised before, let me know, or if it is more appropriate to define strain and other subspecies concepts in other OBO ontologies, please refer me to them...