Change TreeMakerAlg to make the tree in initialize

Right now CP::TreeMakerAlg creates the tree in the first execute function. It doesn't actually gain anything from this as all information required to initialize the tree is known at initialization. Having it this way also precludes using this in workflows where branches should be added in initialization.

As part of this fix I also cleaned up the code a bit

• use Gaudi::Property rather than manual property declarations
• remove unused code (TTree forward declaration and undefined setupTree function)