In particular, it is necessary to turn back DataTree::AddVariable() into a
non-virtual method, because it is called from DataTree's constructor. Enforcing
the absence of leads/lags is now done using a new boolean DataTree::is_static.

Take advantage of the new copy constructor for handling
PlannerObjectiveStatement more elegantly.

Unfortunately it is not possible to implement *move* constructor / assigment
operators, because the reference ExprNode::datatree is not mutable.

This code is broken since the temporary terms array changes. It is probably not
worth maintaining it.

In C++11, it is no longer necessary to put a space between two consecutive ">".

This reverts commit 8bc946c148b3e3321e23bfbace19c91feb2893dc.

(commit lost in merge)
(cherry picked from commit d7db5070e7cf8d1b043062b04d3bd3c4e18ce6c1)

- allow recursive definitions (Closes #554)
- allow a variable to be defined twice, simply emit a warning (Ref #556)

Preprocessor: in steady_state_model block, allow MATLAB functions which return several arguments (closes #37)

Steady state file: in the presence of 'ramsey_policy', no longer check that the declarations are recursive, and use input vector of steady state file as initial values