memory leak while assigning boost::python attributes

I detected a memory leak in O.bodies.erase, when facets were erased (spheres are erased without memory leak!). In the attachment you will find two files, where you can reproduce the bug.

1. Open a terminal with three tabs.
2. Run top or htop in the first terminal tab
3. In the second terminal tab run yade input file "memoryleak-erase-facets.py"
4. Get the pid from first terminal tab (example 31667) and edit "get-memory-info.py"
5. Run "python get-memory-info.py" in the third terminal tab and wait (ca. 5 min) to the end of the calculation.

You will get an output file called "fig-memoryleak-erase-facets.png", where you can see the effect of the memoryleak.

Original bug report by Christian Jakob on 2012-08-24 : https://bugs.launchpad.net/yade/+bug/1041084