Valid FlowNet without additional nodes and removal of well nodes

[olwijn]

It was observed for a large field case with many wells that one (or a combination) of the tube removal options may lead to the removal of well nodes. This will cause an error in the FlowNet workflow. Well nodes should always remain connected.
A somewhat related issue: sometmes FLowNet reports that the created network is not valid and suggest specifying additional nodes as a fix. A valid network should ideally be created also without additional well nodes.