Writing relative displacements from solid solvers

[MakisH]

#236 added the possibility to write absolute displacements, effectively enabling structure solvers (such as solidDisplacementFoam or solids4Foam) to use the adapter.

It would still be nice if the adapter could also write relative displacements (DisplacementDelta). This should hopefully not be so complicated to implement, assuming it is relatively similar to the reading part.