Implement a merge function in the Diagram class

[KGergo88]

Description

The CMDP changes introduced a feature that already stored diagrams can be now updated with new data. At the moment it is implemented in the simplest possible way that the old diagram object will be simply replaced by the new one. This is not an optimal solution and only the updates should be copied.

Todo

• Define different merging rules (e.g. only the new points are added, old ones are reviewed as well, full copy)
• Implement the feature
• Add unit tests for the feature