Feature request: obtain constituents in cluster form

[kpedro88]

Currently, using jet substructure functions that take constituents as input requires the following repetitive pattern:

jetdef = fastjet.JetDefinition(fastjet.antikt_algorithm, 0.8)
    jet_cluster = fastjet._pyjet.AwkwardClusterSequence(array, jetdef)
    constituents = jet_cluster.constituents()
    constituents_cluster = fastjet._pyjet.AwkwardClusterSequence(constituents, jetdef)
    result = constituents_cluster.njettiness()

This could be streamlined as follows: (using the example from #336)

In _pyjet.py:

    def constituents(self, min_pt=0, cluster=False):
        result = self._internalrep.constituents(min_pt)
        if cluster:
            return AwkwardClusterSequence(result, self._jetdef)
        else:
            return result

and then in the user code:

constituents_cluster = jet_cluster.constituents(cluster=True)
result = constituents_cluster.njettiness()

Note from @lgray:

this will require some care with the dask implementation since we have to avoid accidentally sending the client side cluster sequence to a node.