extend particle assign specialization 

- add functor to set an attribute of a particle to their default value
- extend the particle assign definition (set attributes which are not defined in the source particle to their default value)  [discussion](https://github.com/ComputationalRadiationPhysics/picongpu/pull/595#discussion_r22087654)
- remove old usage of `SetValueToDefault` with the new PMacc functor