Make pipeline definition working

[denisri]

The Node and Pipeline classes have been simplified, but there is now a confusion between ProcessNode and non-process Node: in non-process Node instances, plugs are fields on the Node instance. For ProcessNode instances, plugs fields/values are actually in the underlying process. Node methods get_plug_value() and get_trait() have disappeared, but then we don't have a transparent way of getting / setting fields and values. For instance in Pipeline.export_parameters(), we have:

        # Make a copy of the field
        source_field = node.process.field(plug_name)

assumes the node has a process inside, which doesn't work for non-process nodes (switches etc) then, later in the same function:

        # Propagate the parameter value to the new exported one
        v = getattr(node, plug_name, undefined)

which, on the contrary, assumes that plug values are on the Node instance itself, which will work for a switch, but not for a process... So:

• either we have to re-introduce Node.get_plug_value() and a replacement for Node.get_trait() (Node.get_field() or Node.get_plug_field() for instance), but the new undefined value behaviour (which throws exceptions) should be catched by explicit try.. except blocks around, because here we cannot use getattr(node, plug_name, undefined).
• or we can think of another implementation: let Process inherit Node. Both share many things, and process/non-process nodes thus have an inconsistent shape, that we used to hide by reimplementing access functions to plugs: process nodes were delegating to their underlying self.process, while other nodes were using their own traits. If Node was the base class for Process, this delegation would no longer be needed. As Process instances are most of the time used inside a Pipeline, it would lighten the pipeline objects structure. In non-pipeline implementations, there would be "plugs" on the process which would not be connected, but is it a problem ? Plus, it would remove the need for Pipeline.pipeline_node, and solve (partly only) a number of cyclic references problems between Node and Process when deleting a pipeline that we unsuccessfully tried to solve using weakref and weakproxy cuisine.

[denisri]

So I will try implementing the Pipeline(Node) inheritance. I will need to move the pipeline_nodes.py file since it will be used by Process.