LArG4Barrel: const fixes for CryostatCalibration

Frank Winklmeierrequested to merge
fwinkl/athena:larg4barell3 into master
  • Use const-initialization of volumeMap in CalibrationCalculator by moving the initialization code into a Meyer's singleton.
  • Reduce usage of static in favor of constexpr where possible.

Relates to ATLASRECTS-7235.

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