Add accessor to ServiceHandle to allow non const access to service, even from a const handle

See the discussion in gaudi/Gaudi!225

I am not completely sure what I think of this. I am not completely sold on the idea, but can see Services are different beasts to tools. 

I submit this for discussion and testing by Atlas( @leggett I've tested it in as far as Gaudi compiles but as in LHCb we have no use case that requires it, cannot go much future. It would be good if this MR could be tested against Atlas *before* merging this time ;))

See merge request !233