Builds one node of a gbm model tree
build_node_gbm(tree, categorical_lookup, leaf_val_type, whichNode,
valueNeedsTag, dataLevels, fieldTypes)tree object
splits used
a character representing an avro type when a value is returned at a leaf node
left or right node for categorical_lookup
flag for whether node needs label
levels of data
type of fields
PFA as a list-of-lists that can be inserted into a cell or pool