Get the display species-sets from ncbi_taxa_name

The list of species sets is now stored in the ncbi_taxa_node table.
Only the set of low-coverage species has to be fetched in a different
way.
Simplify the "genetree_display" logic

The "genetree_display" key is there by construction of the hash.
There is no need of checking it.
Merge pull request #15 from muffato/feature/compara_species_sets

Compara species sets