Sort PFO containers by pT

The PFO containers were not explicitly sorted, which potentially causes problems in optimisation where reducing the number of operations on the input objects may change the ordering of the intermediate containers and hence make it more difficult to validate changes. This MR implements pT-sorting for all the PFO containers, both neutral and charged.

As usual I threw in a few minor improvements, like making the algs reentrant and therefore all methods const, plus some clarification of the code.