Small refactoring adding a _get_locations method on product.mrp.area
which by defaults delegates the computation to the related mrp.area.
This enables extending a few things related to locations at the
product.mrp.area level.
Change the way `_get_locations()` work: don't return the list of child
locations, only the top-most locations, and then use the `child_of`
operator in the code that looks for locations.
* You can know integrate with different forecasting mechanisms.
As a consequence integration with stock_demand_estimate is moved
to a new module (mrp_multi_level_estimate).
* As agreed by the authors, the module is re-licensed to LGPL-3.
