Fix bug: double click on composite node in package view failed

Error was:
Traceback (most recent call last):
  File "${OPENALEA_SRC}/oalab/src/openalea/oalab/package/treeview.py", line 40, in mouseDoubleClickEvent
    cat, fn = applet.add(applet.project(), obj.name + '.wpy', obj, category='model', dtype='Workflow')
  File "${OPENALEA_SRC}/oalab/src/openalea/oalab/gui/container.py", line 342, in add
    data = project.add(category=category, filename=filename, content=code, dtype=dtype)
  File "${OPENALEA_SRC}/core/src/core/project/project.py", line 230, in add
    return self.add_item(category, obj, **kwargs)
  File "${OPENALEA_SRC}/core/src/core/project/project.py", line 336, in add_item
    data = self._add_item(category, obj, **kwargs)
  File "${OPENALEA_SRC}/core/src/core/project/project.py", line 305, in _add_item
    data_obj = DataFactory(new_path, mimetype, dtype=dtype, default_content=content)
  File "${OPENALEA_SRC}/core/src/core/service/data.py", line 123, in DataFactory
    f.write(default_content)
TypeError: must be convertible to a buffer, not CompositeNodeFactory
Factorize IPython user_ns management in Models
Rename namespace functions

_save_ns => _push_ns
_reload_ns => _pop_ns
_save_init_var => _populate_ns
Merge pull request #94 from gbaty/clean_model_rebased

Clean model rebased
Project settings are now managed in manager instead of widget
Fix bug if LPyModel is called from dataflow
Move tests from module to test directory
Merge pull request #97 from gbaty/minor_bugfix

Minor bugfix
Add/Update some headers
Merge pull request #98 from gbaty/header

Add/Update some headers
All applet plugin follow standard plugin convention: __call__ return applet class.
To add widget in mainwindow and fill menus, just call "graft(mainwindow)" method.
Improve PluginManager.instances, get_applets
Move oalab.plugins interfaces to core.plugin
Real oalab plugins like applets, paradigms, remain in oalab.plugins
All applet plugin now follow same API
All applet class can be instantiated with zero parameters
get_applet service is now based on PluginManager
Add PluginManager in session debug mode
Improve plugin manager and simplify labs
Adapt to new PluginManager API and plugin services
Applet service use new plugin services