Add const keyword to fix const-correctness issue of DCMathSegmentMaker.

Add const keyword to fix const-correctness issue of DCMathSegmentMaker.

http://atlas-computing.web.cern.ch/atlas-computing/links/distDirectory/gitwww/experimentalWebArea/nicos_web_areaconstmaster_Athena_x86_64centos7gcc8opt64BC7G8AthenaOpt/NICOS_Log_2019-06-08T1008/MuonSpectrometer_MuonReconstruction_MuonSegmentMakers_MuonSegmentMakerTools_DCMathSegmentMaker.html

+ MuonSpectrometer/MuonReconstruction/MuonTrackMakers/MuonTrackMakerTools/MuonTrackFinderTools
+ MuonSpectrometer/MuonReconstruction/MuonSegmentMakers/MuonSegmentMakerTools/MdtSegmentT0Fitter
+ MuonSpectrometer/MuonReconstruction/MuonSegmentMakers/MuonSegmentMakerTools/DCMathSegmentMaker
+ MuonSpectrometer/MuonReconstruction/MuonSegmentMakers/MuonSegmentMakerInterfaces
+ Tracking/TrkUtilityPackages/TrkDriftCircleMath

were compiled by

cmake -DATLAS_PACKAGE_FILTER_FILE=../package_filters.txt  -DATLAS_ALLOW_TOOLHANDLE_NONCONSTNESS=OFF ../athena/Projects/WorkDir
make