Don't bother with otm_code for migrations

Migrations will come with predetermined ITreeRegion codes, which should
be written into the migration as ITreeCodeOverride records.

Doing code searches is a second-best method for producing calculable
ecobenefits for species.

Only one of these methods should be used.
Bail out of migrations if species could conflict
Merge pull request #1619 from steventlamb/topic/fix_migrator_species_duplication

Topic/fix migrator species duplication