Mark a task as a milestone if it is a leaf node and cannot be scheduled
* org-taskjuggler.el (org-taskjuggler-compute-task-leafiness): Compute
the leafiness of a node.
(org-taskjuggler-open-task, org-export-as-taskjuggler): Mark a node as
a milestone if it is a leaf and has no effort.
Mark a task as a milestone if it has neither an effort, a duration, an
end or a period. This is needed for tj3 export as tj3 fails to compile
the file if there is a leaf node with no computable end date. tj2
happily ignored the situation, but TJ3 throws an error.