Reworkign the Cluster Implementation, remove APIX specific code

This pull request is not intended to be merged as of now. It removes APIX specific code and reworks the cluster implementation.
Two new pixel types have been introduced: EUTelGenericSparsePixel and EUTelGeometricPixel. Access to sparse data is provided via the EUTelTrackerDataInterfacer which is intended to replace the EUTelSparseData(2)Impl. It also provides the interfacing capabilities for any cluster, reusing centrally managed code. More info will follow ..