Add a class to describe a parton in parton shower
Provide exception specification for physics objects classes

Also change indentation in the header.
Change indentation in PECReader
Specify size of buffers in PECReader using a static const instead of a
proprocessor marco
Add an interface for access to partons from parton shower

At the moment there is no implementation, only a placeholder.
Add missing PDG ID to the ShowerParton class
Implement access to partons from parton shower