Use AtlasFieldCacheCondObj instead of IMagFieldSvc

Nicolas Koehlerrequested to merge
nkoehler/athena:rec5468 into master

Hi,

this MR moves from using IMagFieldSvc towards using the thread-safe AtlasFieldCacheCondObj in MM_DigitizationTool. It fixes ATLASRECTS-5468.

Best, Nico

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