Implement map<Identifier,DetectorElement> (was ACTS-302)

Original author Julia Hrdinka @jhrdinka

In order to do assign hits (where a ID of the detector element is known) to a corresponding Acts::Surface it would be useful to have a map which connects surfaces to these IDs. Hence, a map should be created which contains all DetectorElements (from which one can access the Surface) of the geometry and connects them with their ID. This map should be accessible from the TrackingGeometry.