PileUpTool property clean-up

This merge request aims to update the syntax of a number of PileUpTool implementations.
The main changes are to:

• move all member variable initializations to the header files, including switching from using declareProperty(...) in the constructor to using Gaudi::Property declarataions in the header.
• use ATH_CHECK macros in the initialize method.
• set up the PileUpMergeSvc in a more standard way.