Test1

@svensson ppfporttask only has optional input which means it is recognized as a start node when it has no non-conditional predecessors. This MR: make the optional input required.

This would happen to any node with only optional input and no predecessors or only conditional predecessors. It has nothing to do with submodels or ppfporttask. So maybe a better solution is to add a node attribute that explicitly says this node is not a start node?

There might be an automatic way by analyzing the graph for loops, not sure, it won't be easy ...

Or we enforce that a start node should not have any predecessors. Which means we cannot deal with graphs like this: https://gitlab.esrf.fr/svensson/workflow_concepts/-/blob/master/examples/running_cyclic_pipelines.ipynb

Consequently we would only support a subset of all "directed graphs" which makes the specification less clear.