egammaAlg : start preparing for re-entrant part 1

• 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.