Allow usage of a custom 'SampleMetaData' type

E.g. By allowing the user to extend the base SampleMetaData class via inheritance.

I suppose it would make sense to as provide as well a factory, which can be extended by the user, so that having a custom SampleMetaData is opt-in.

Background: E.g. the Digitizer Class requires passing a custom 'status' bit-enum to the client.