Fix warnings

Marco Clemencicrequested to merge
prepare-gaudi-408 into master

gaudi/Gaudi!408 (merged) implies that more warnings are exposed. This MR fixes a few issues:

  • drop dependency on GaudiKernel/xtoa.h
  • replace deprecated auto_ptr with unique_ptr
  • fix a couple of misc warnings
Edited by Marco Clemencic

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