Update MdtSlewCorFuncHardcoded for AthenaMT

This function had caused a crash as reported in ATR-23530. Rather than create a mutex as suggested, move initialization of LUT to the class constructor, which also avoids having to check of the LUT is initialized every time the correction function is called. In addition, changed to LUT from a map to vector which should have faster access. RunTier0Tests.py ran without any errors after change.