Domain changes to allow delete of Taxons.

Hi there,

I made some changes to the domain model (Taxon and TaxonLink).  I added hasMany relationships to Taxon and made TaxonLink "belongsTo" Taxon instead of just including a reference.  I needed to do this because I wanted to be able to delete empty Taxon objects, but delete was cascading to the parent instead of the other way around.  For info on this problem, see:

http://stackoverflow.com/questions/5472452/grails-gorm-self-referential-belongsto-deletes-opposite-direction-from-expected

Please feel free to let me know if you have any questions.

Thanks,
Jim