- Now Dof holds a weak_ptr toward its Node, which itself gets one toward its NodeBearer.
- Whether a Dof belongs to a NumberingSubset is now handled by a test upon its Node.
- Dof program-wise index by numbering subset are now mandatory to fill processor-wise index by numbering subset - this is to avoid invalid state of the object, which is quite complicated to build (part of it before partitioning, and part
of it after).