major revision of HLTTPCDigitReaders

major revision of HLTTPCDigitReaders
- corrections in the interface functions added in rev. 29023 to
  get the RCU trailer -> now passing by reference
- speedup of DigitReaderPacked by using a singleton of the AltroRawStream
  and thus avoiding of multiple allocations of TPCMapping aso
- speedup of DigitReaderPacked in sorted mode by using correct array bounds
- corrected implementation of bulk read mode in DigitReaderPacked
- code cleanup in DigitReaderPacked