Add SliceMeshStorage::getZSeamOrigin().
Add SliceMeshStorage::getZSeamOrigin().

This returns a point which is the middle of the mesh's bounding box offset by the
user specified z seam location.

alephobjects/CuraEngine,ROBO3D/CuraEngine,Ultimaker/CuraEngine,alephobjects/CuraEngine,alephobjects/CuraEngine,ROBO3D/CuraEngine,Ultimaker/CuraEngine,ROBO3D/CuraEngine