feature request: need `squash()` for MetaGraph and MetaDiGraph

[zhangliye]

Need squash() for MetaGraph and MetaDiGraph

[simonschoelly]

I think such a feature would make sense.

But such a feature should only squash the topological information (i.e the wrapped graph) and not any metadata.

Dictionaries with meta data should be copied. What is not exactly clear here is, if the elements of the dictionaries should be copied with copy or deepcopy. Maybe an additional kwargs flag would be useful here?