Extend AlgExecState with an "executing" state

The AlgExecState, accessible via the IAlgExecStateSvc, now records if an algorithm is currently being executed. This required some interface changes to the AlgExecState, i.e. replacing the boolean [set/is]Executed() with a enum-based [set]execState(). Clients have been updated accordingly. This also opens the door for further algorithm states if necessary later.

cc @leggett