We follow the advice given by Josuttis in his book about move semantics.
Deleting those member fuctions can be a bug if we want to allow copy semantics,
because overload resolution will no longer fallback to the copy
constructor/assignment operator when given an rvalue.
In particular, this explain why it was not possible to delete the move
assignment operator of the StaticModel class.
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.
Sébastien Villemot
Houtan Bastani
Stéphane Adjemian (Charybdis)
