[InterventionalRadiologyController] Improve init checks to avoid potential crashes

[epernod]

• If no instrument is given as input, the init method was searching for a single instrument using GetContext.

WBeamInterpolation * wbinterpol= context->get< WBeamInterpolation >(BaseContext::Local);
m_instrumentsList.push_back(wbinterpol);

The pointer was not tested so if no instrument were present in the scene, the IRCtrl was initialised with a null pointer.
Change this method to directly fill the vector with all corresponding component found.

• Add a warning at init if the number of dof inside the current mechanicalObject is really low compare to the number of beam that can be deployed in the scene. this doesn't really fix [Crash] in InterventionalRadiologyController if nbBeam > nbDof #70 but at least warn user.