Migrate GlobalChi2Fitter to use new conditions

The GlobalChi2Fitter tool was previously interacting directly with the
old-style magnetic field service. For reasons of thread safety, this is
no longer the recommended way of retrieving information about the
magnetic field. Instead, we should access this information through
magnetic field cache objects from the new conditions architecture. This
commit replaces the old-style magnetic field code with new-style code by
adding a magnetic field cache data member to the fitter's internal cache
class, adding logic to initialise this magnetic field cache on an
event-by-event basis, as well as by modifying the logic that previously
accessed the service directly.

This commit resolves ATLASRECTS-5441.