Add missing import
Add a new method for formatting an expression tree as an actual binary expression.  This requires one input which is a mapping of operators to a numerical precedence so that the correct order of operations can be determined.  This ended up looking a lot like ExpressionTree.evaluate, which might be able to be used to implement this with a little refactoring.
Reformat format_ascii_tree a bit, supporting numerical labeling of leafs.  This method still needs more work though :/
Initial work on better repr for CompoundModel classes.