Port openalea.core to Python 3

There is no setter method to change the evaluator for a workflow but change the eval_algo variable of the composite node.
Plus the list of available evaluator is not user friendly to get :
from openalea.core.algo import dataflow_evaluation as df
df.evaluators

Application crash when deleting a edge:

File "/home/fabrice/Documents/Openalea_Dev/core/src/openalea/core/node.py", line 643, in set_input_state
index = self.map_index_in[index_key]
KeyError: 1

To reproduce:

• place one widgets with 2 output like hydroroot.compute_surface
• place a second widget like float
• link the float output to the compute_surface input
• delete the edge

The reason:

• in openalea.core.compositenode.CompositeNode.remove_edge port = self.port(target) is not the correct port widget

core/src/openalea/core/package.py:44: DeprecationWarning: the imp module is deprecated in favour     of importlib; see the module's documentation for alternative uses
import imp

In the method register_package use importlib rather than imp

Should we have from openalea.visualea in core. They appear in instantiate functions in:

• src/openalea/core/node.py, twice
• src/openalea/core/compositenode.py, twice
• src/openalea/core/data.py, once

Should not be a problem, because those lines should be called only when using graphical interface, I guess, so when we use visualea.

The library may lead to problems. For example six.string_types gave a tuple and not the correct type in the file core/src/openalea/core/interface.py.

This error message is raised in openalea.visualea.node_widget.SignalSlotListener.call_notify. The cause is the obsolete PyQt4 code:

self.qobj().emit(QtCore.Signal("notify"), sender, event

I tried what is given in the documentation but I was enable to get something working. I still got the message has no attribute 'emit'.

port to Qt5 not done