Add two methods that are local to tests, `registerNodeDesc` and
`unregisterNodeDesc`, which handle the registration and unregistration
of `NodePlugins` from a node description. This reduces the amount of
code in tests whenever `NodePlugin` need to be instantiated prior
to their registration and so on.
Inputs and outputs Attributes are now declared in two separate lists
* no more isOutput field on Attribute description
* guarantee attributes order (was not the case in Python2 with class attributes)
* Attribute's name is now part of its description
Note: for now, input and output Attributes are still stored in a single model inside a Graph.
