egammaAlg : start preparing for re-entrant part 2

• For now add an execute_r(const EventContext& ctx) const
• Make the standard execute() call it
• At some stage execute_r will become execute when the Alg will become re-entrant
• Also tidy up a bit things

There was a couple of methods that were not marked const while the could also helps to straighten out the ctx propagation.