fix runtime warning while python conversion

[jihoonl]

Fixing python conversion warning using register_ptr_to_python

/home/jihoonl/research/ros/ecto/devel/lib/python2.7/dist-packages/ecto/__init__.py:30: RuntimeWarning: to-Python converter for boost::shared_ptr<ecto::tendril> already registered; second conversion method ignored.
  del os_path
/home/jihoonl/research/ros/ecto/devel/lib/python2.7/dist-packages/ecto/__init__.py:30: RuntimeWarning: to-Python converter for boost::shared_ptr<ecto::cell> already registered; second conversion method ignored.
  del os_path
/home/jihoonl/research/ros/ecto/devel/lib/python2.7/dist-packages/ecto/__init__.py:30: RuntimeWarning: to-Python converter for boost::shared_ptr<ecto::py::cellwrap> already registered; second conversion method ignored.

As proposed in BVLC/caffe#4069

[vrabaud]

Thx !