Changes in test structure, not going to test the outputs from
toFirstOrder separately.
Making the treeVar power method call the static bivariate() method.
Found out that a new property indicating whether expression has
terms in it makes the tree expansion more robust.
Further simplification of the expandTree method.
Correct sorting of output from first order conversion.
An big test of the first order conversion capabilities.
Adding documentation for new treeVar tree property.