The segfault happens when the component is located in a node without
a topology and a mechanical object. Then the 'draw' function is crashing.

…MForceField

The component was searching the topology in the context which was:
- implicit
- constraining the organization of components the scene graph.

To solve this I added a SingleLink that allows to specify the topology.
If the link is not set then the existing behavior is used.

The checked ifdef is not valid and not following the SOFA way of doing
things. So I fixed it.

…pology.

… mechanicalObject