Do not use GaudiAlg

[jmcarcell]

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

Changes proposed in this pull-request:

• 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]

Locally (using the Key4hep nightlies) this builds for me, there doesn't seem to be any ctest tests so I can't run them but behavior should be unchanged.

[jmcarcell]

This is another one that would be nice to have merged for building @kjvbrt or @BrieucF (and the last in the batch) I believe.

[kjvbrt]

Hi @jmcarcell can you rebase on top of #39 ?

[jmcarcell]

Done

[kjvbrt]

Thank you :)