Given all the nodes from one level, join_node evaluates all possible
pairs one by one and decides whether or not to join them based on the
similarity between the estimated ability levels and the density functions.
If a pair of nodes is joined, the density function of the resulting node is
the mean of the density functions of the joined nodes and the confluencies
are summed.
join_node(nodes, level, limit, tol, inters)list of node lists. Every node list must contain the ID of the node, the vector of density function values of the evaluated abiity levels, the vector of previous items, the estimated ability level and the node confluency
level of the CAT decision tree
maximum number of level nodes
minimum distance between estimated ability levels to join two nodes
minimum common area between density functions in the nodes of the evaluated pair in order to join them
A list of node lists. This list is the input list updated with the joined nodes