Fix failure of SimKernelMT with -DATLAS_ALLOW_TOOLHANDLE_NONCONSTNESS=OFF of ISF_Algorithms package

Fix failure of SimKernelMT with -DATLAS_ALLOW_TOOLHANDLE_NONCONSTNESS=OFF of ISF_Algorithms

Make returned reference of identifySimulator method const.

http://atlas-computing.web.cern.ch/atlas-computing/links/distDirectory/gitwww/experimentalWebArea/nicos_web_areaconstmaster_Athena_x86_64centos7gcc8opt64BC7G8AthenaOpt/NICOS_Log_2019-08-10T2138/Simulation_ISF_ISF_Core_ISF_Algorithms.html

https://indico.cern.ch/event/757141/contributions/3138783/attachments/1714912/2766189/2018.09.13_constGaudiHandle.pdf