bugfix: copy() doesn't copy node_ids any more
bugfix: copy() doesn't copy node_ids any more

The new method is actually much faster as the tree is only traversed once,
and the restricted copy is made on the fly.

Ensembl/ensembl-compara,Ensembl/ensembl-compara,Ensembl/ensembl-compara,Ensembl/ensembl-compara,Ensembl/ensembl-compara,Ensembl/ensembl-compara,Ensembl/ensembl-compara,Ensembl/ensembl-compara