Merge OUTSIDE/demo_data and INSIDE/tvb_data

TVB_Distribution /tvb_data/….site-packages/tvb_data is the location added
to the PYTHON_PATH (when starting python from TVB_Distribution).
This is the location from where ipynb tutorials are loading their data
(e.g Connectivity(load_default=True)), as it can be used in python
imports.
From this location also DefaultProject is being loaded, and before the
release I tried to verify that every piece of data referred from
tutorials is being correctly copied there.

TVB_Distribution/demo_data contains only few pieces of data. The
intention was to have it easily accessible by GUI users when they try an
importer from web interface.

As it raises this question, maybe we should rethink this decision (which
was taken so long time ago), to avoid possible confusions.
Possible solutions for the next release:

• create a link on top level, towards the site-packages/tvb_data, or
• add the top level in PYTHON_PATH directly (we should test if this can
  work with py2app as well)