Model composition in CropML

[pradal]

Add the composition of models in xml/dtd. The composition follows the composite design pattern.

A model is either:

• a model uint(atomic)
• or a model composite , that is a model that is composed of models ( unit or composite)

The composition is represented as a directed port graph of models:

• Vertices are the different models that form the composition.
• Ports are the inputs and outputs of each model.
• Edges are directed and connect one output port to an input port of another model.

A description of such workflow or composite model can be found at https://github.com/openalea/wlformat

[pradal]

@cyrillemidingoyi Close when done.

[cyrillemidingoyi]

Done