Modify semantics of ExprNode::countDiffs()

Previously, this function was counting the total number of diff() operators in
an expression. But this is not very useful, and is potentially misleading,
because in practice we use this function to compute the maximum lag on
variables in levels.

This function now returns the maximum number of nested diffs.

For example, on diff(x)+diff(diff(y)), this function was returning 3, and it
now returns 2.