Use local instances of MagneticFieldGridReader in MagneticFieldSvc

Fixes a minor annoyance with the MagneticFieldSvc which is messages sent from it were not respecting changes made to the message formatting. This was due to the timing of when the Reader instances where constructed. This MR fixes this by instead of having a single cache instance, use local instances instead. As the reader has no cached state, and is cheap to construct, this has no overhead.