Add an accessor to shared pointer to CollisionGeometry member.

  - classes CollisionObject and ContinuousCollisionObject,
  - declare former accessors as deprecated.

I think that returning a pointer to a member of type shared pointer is not a good idea. If the user initializes his own shared pointer with the result of CollisionObject::getCollisionGeometry(), the object will be deleted twice. I quickly checked with the following code using valgrind.
```
#include <boost/smart_ptr.hpp>

class A {
public:
  A (int a) : a_ (a) {}
  int a_;
};

int main ()
{
  A* a = new A(1);
  boost::shared_ptr <A> a0 (a);
  boost::shared_ptr <A> a1 (a0.get ());
}
```