Reorganize MeshNodeBoundary: objects can no longer be built manually

One now must use getMeshNodeBoundary which checks if the boundary exists. This also concerns

• MeshFlatNodeBoundary (getMeshFlatNodeBoundary) and
• MeshLineNodeBoundary (getMeshLineNodeBoundary)

assigned to @delpino

enabled an automatic merge when the pipeline for 65b43282 succeeds

mentioned in commit f132852c

merged