Add test for ctree2sas
Load package and use namespace

* checking R code for possible problems ... NOTE
btree_criteria: no visible global function definition for ‘nodes’
btree_left: no visible global function definition for ‘nodes’
btree_prediction: no visible global function definition for ‘nodes’
btree_right: no visible global function definition for ‘nodes’
ctree2sas: no visible global function definition for ‘nodes’
gbm2sas.descend: no visible global function definition for
  ‘pretty.gbm.tree’