Move from Python 2 to Python 3

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• Create a conda package
• Upload it to openalea3 channel

1. Complete the package description in the README.rst and in the project description.
2. Add keywords

All simulations with existing roots give same results in Py2 and Py3.
This is not the case with root generation with a given seed. It seems that it comes from the module "random". For instance, with the same seed, random.expovariate() or random.randint() do not give the same result. I tried to add the argument "version" in random.seed() without any change.
WIP

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2. Add a badge to link the README with the doc

A simple tutorial is needed with explicit parameters, 3D visualisation using oawidgets and explanations.
See openalea.rtfd.io