Return type should be a ClusterSequence

[graeme-a-stewart]

For post-processing after the initial clustering, it is really necessary to have the history of how clusters were merged and access to all of the intermediate merged jets.

Therefore we need to expose the ClusterSequence in the package and make this the actual return type.

Functions such as inclusive_jets(::ClusterSequence) are then used to (in this case) return all of the final state jets.

Fixes #29