Check to ensure there is only one outlet #12
Description
RORB requires a single outlet. This is represented by setting the 'out' attribute of the confluence to one. If there are multiple confluences with the attribute set to one, the pyromb will select the out location as the first feature it encounters with the out attribute as one.
To avoid this confusion, pyromb should check that there is only a single 'out' attribute set to one, if not throw an error and advise the user.