improve detailed desc of class2tree

fix #850 remove line of code in ncbi_downstream that wasnt needed and caused an error when an object was empty

#847 change prune_too_low, add new param ignore_missing_rank

#848 change downstream and children when db="worms" to paginate to get all results

#848 allow marine_only parameter to be passed down to worrms::wm_children in various fxns

fix #847 add epifamily rank to rank_ref_zoo dataset, bump version