Commit formally makes VectorLaplaceOperator smoothable

The VectorLaplaceOperator class now publically inherits from
- GSSmoothable
- GSBackwardsSmoothable
- SORSmoothable
- SORBackwardsSmoothable
and implements the associated pure virtual methods. The
implementation in turn checks that the diagonal sub-operators
have the corresponding interface and delegates work to them.

Note: Of course a P2ElementwiseVectorLaplaceOperator is still
not truely SORSmoothable. However, that will only be recognised
on the sub-operator level.
29 jobs for mohr/VectorSmoothables-mc in 136 minutes and 24 seconds (queued for 2 seconds)