Remove internal data from PseudoJet

[graeme-a-stewart]

The PseudoJet struct is extremely useful and works very well in this package. However, it has one defect when exposed publicly, viz. a data member which is completely for the internal use of the N2Tiled algorithm, _cluster_hist_index.

This should be removed to make the struct clean for export and an internal index added to be used during tiled reconstruction.

[graeme-a-stewart]

Actually, on further investigation, this is really needed, because this provides the link into the merging history which is absolutely required for fine control of the reconstruction and downstream analysis. So this should not be removed, but work needs to be done so that the first class reconstruction object is a ClusterSequence a la FastJet.