Improve IMeasurementProvider interface

• remove 'measurement' call which forces, for each measurement, a seperate lookup in the event store, followed by a (linear!) search in the cluster/hit container for the corresponding hit/cluster
• update users of the IMeasurementProvider interface
• update implementations of the IMeasurementProvider interface, reducing the number of lookups to one per type of hit per track (unfortunately, the linear search inside the hit/cluster container is still there for the time being)