Extend CopyMeGraph model by save_graph_entity_properties function

Description

In order for the CopyMeGraph model to cover all relevant graph utilities, it should be extended by the use of the save_vertex_properties (`save_edge_properties) function. This way of writing graph data is recommended for dynamic graphs.

Also, the structure of the graph should be written (such that it can then be loaded as a GraphGroup). This is needed in !11 (merged) where a networkx.Graph is created from a GraphGroup.

Proposal

• Create two GraphGroups, one *_static and one *_dynamic, and use the save_vertex_properties function to write into the latter
• Additionally write out the graph structure, i.e. _vertices and _edges, in particular create a data structure that can be loaded as GraphGroup (needed in !11 (merged))
• Add documentation / more detailed comment about the writing of graph data

How to test the implementation?

Related issues

Meta-task: #...