Tidy up PhotonHitFPD code

Standardise layout/whitespace with other code and add some comments.
Remove unused PhotonHitFPD members and methods

This includes all SMD-related members
Use single constructor for PhotonHitFPD

Replace separate default constructor and constructor with arguments
with a single constructor with default arguments.

Remove Clear() method as it is no longer used.
Make PhotonHitFPD a struct

As it now has only simple data members and very little functionality,
change PhotonHitFPD from a class to a struct. Even though there is no
functional effect here, it follows the convention of using struct for
simple data-only objects.
Remove PhotonHitFPD inheritance from TObject

This class is never placed in a ROOT container, and never makes use of
any other functionality requiring it to inherit from TObject.

Reinstate Clear() method that actually was required despite being
previously removed.

Completes reduction of PhotonHitFPD code to close #6.