readonly interface and better correctness test for ScopeTreeTimer

* Made eventlist access private for ScopeTreeTimer and added a public
readonly interface
* Enhanced ScopeTreeTimer Correctness test to also check for correct
number of enter/exit events
* Renamed boolean member of TimerEvent, to improve semantics (true: we
are in the scope)
const placement and namespace fixes

* Moved const keyword consistently after type specifier
* Fixed missing namespace import for duration_cast