Refactor code for collecting variables appearing in expressions

- rename ExprNode::collectVariables in ExprNode::collectDynamicVariables
- new ExprNode::collectVariables: same as above, but without lag information
- remove ExprNode::findUnusedEndogenous: essentially redundant with the above)
- remove ExprNode::collectModelLocalVariables: idem
Add a warning when some exogenous are not used in the model
Forbid the use of trend variables in expressions outside the model block
Fix config of warnings in @dseries/cumsum tests