TrkVKalVrtFitter+TrkVKalVrtCore: Update for thread-safety checker const changes.
The thread-safety checker is getting more strict on const checks. Fix resulting warnings.
The methods of AtlasFieldCache that retrieve the field are non-const. Hence, any methods that IVKalState and can access the field have to take a non-const IVKalState.
Edited by Scott Snyder