Fix edge metadata key when cloning

[silas-martens]

When merging edge metadata, clone() returns a std::unique_ptr<MetaData>. Consequently, template argument deduction for addEdgeMetaData() selects T = MetaData, causing T::key to resolve to MetaData::key ("BaseClass") rather than the key of the dynamic metadata type.

I noticed this when cgmerge2 changed stored my previously correct metadata under the "BaseClass" key.

I fixed this, by using the virtual getKey() method when inserting edge metadata.

[pearzt]

LGTM. I'm surprised that this was not caught by tests. Could you take a look into the tests and make sure this is covered?

[TimHeldmann]

It is a shame, that C++ does not support covariant return-types for unique_ptr specializations.
So I think this fix is valid, if we do not want to go back to raw pointers for the cloning functions, or want to introduce an additional CRTP layer to the metadata registry.

If you are able to add a test, this looks good to me