Improve consistency of ExprNode methods for calculating max lead/lag

- ExprNode::maxLag() and ExprNode::maxLead() now take into account exogenous
  deterministic variables, for consistency with M_.maximum_{lead,lag}

- ExprNode::maxLag() no longer behaves as if diff() operators were
  expanded (i.e. it now returns 1 on diff(x(-1))), for consistency with
  maxEndoLag() and maxExoLag()

- New ExprNode::maxLagWithDiffsExpanded() method, that behaves as maxLag() used
  to behave (except that it also takes exogenous deterministic into account)