ns-3: align usecorepositions() with session object mobility

```
usecorepositions() is member of CoreNs3Net; this is for alignment with other
CORE wireless networks where the position is associated with the wireless
interfaces.

The command to make ns-3 take the GUI position updates is:

wifi.usecorepositions()

while the command to make ns-3 run its own mobility is instead part of the
Session object; e.g.:

session.setuprandomwalkmobility(...)

These probably should be aligned; the proposal for now is to create a new
Session method: usecorepositions(), and keep the old CoreNs3Net one for now.

```

Original issue reported on code.google.com by `tomh....@gmail.com` on 15 Feb 2013 at 11:42