Do not use GaudiAlg, use Gaudi::Algorithm instead

[jmcarcell]

These are the changes needed not to use GaudiAlg, which will be removed (maybe soon) in Gaudi.

BEGINRELEASENOTES

• Do not link to GaudiAlgLib
• Add the necessary changes to the algorithm, use Gaudi::Algorithm, make execute() const and add a const EventContext&
• Add mutable to some elements that are changed; add const to some functions so that they can be called inside execute(const EventContext&) const

ENDRELEASENOTES