New species_tree() method in GeneTree
Slight reorganisation of the species-tree methods in BaseRunnable
Use GeneTree::species_tree()
Use the gdb2stn mlss-tag that has been created by MakeSpeciesTree
Merge branch 'species_tree'

* species_tree:
  Use the gdb2stn mlss-tag that has been created by MakeSpeciesTree
  Use GeneTree::species_tree()
  Slight reorganisation of the species-tree methods in BaseRunnable
  New species_tree() method in GeneTree