Minor fix to previous.
Turns out testing bivariate method is possible with the same methodology.
Tidying up the TREEVAR classdef file.
Proper testing for treeVar.printTree(), making the method
output a string, rather than printing directly with fprintf.
Adding a test method for printTree. The method now returns a
string, rather than just printing directly to command window
with fprintf.
Adding a test method for the plotTree method.