Adds a variable to the model.

def add_var(self, var):
        """ Adds a variable to the model.
        """
        if var.name in [v.name for v in self.vars]:
            logger.error("Variable set named '%s' already exists." % var.name)
            return

        var.i1 = self.var_N
        var.iN = self.var_N + var.N - 1
        self.vars.append(var)