Migrate GlobalChi2Fitter to in-class Gaudi properties

As far as I know, using in-class Gaudi properties is the preferred and modern way of declaring properties for a tool. The GlobalChi2Fitter is currently still using the older in-constructor style properties, and this commit changes this to the new style. Besides being more modern, idiomatic Gaudi, it also means the properties, their config-level names, and their default values are now in a single place is which a lot easier to wrap the head around. The only property not migrated was a service handle, because of some runtime parameters being passed to its declaration statement.