Do not use GaudiAlg

[jmcarcell]

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

Changes:

• 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
• Change GaudiTool to AlgTool

[jmcarcell]

To be able to keep building the nightlies with Gaudi without GaudiAlg a review here would be nice @BrieucF or @kjvbrt.

[kjvbrt]

Thank you @jmcarcell :)