Small improvements to HPD image fitting

This change does not alter the behaviour of the monitor in any significant way, and this monitor has no impact at all on the reconstruction performance itself.

The main change, other than some C++11 clean ups, is to create the HPD image histograms on demand if required. This closes a potential loop hole (that so far as never been hit and probably never would be) where the list of HPDs changes between initialisation and the event loop.

This algorithm is not run at all in HLT2, and only as a monitor in Brunel. Its primary use is as the HPD image calibration task running at the pit.