Partially fix const correctness issue of UnassociatedHitsGetterTool.

Partially fix const correctness issue of UnassociatedHitsGetterTool by adding const to its methods get and releaseObject. These methods have to be const because DerivationFramework::IAugmentationTool::addBranches() is const.

Since Trk::PRD_AssociationTool::addPRDs( const Trk::Track& track ) and Trk::PRD_AssociationTool::reset() are still non-const, the const correctness issue is still there.

http://atlas-computing.web.cern.ch/atlas-computing/links/distDirectory/gitwww/UPGRADE_INTEGRATIONWebArea/nicos_web_areaconstmaster_Athena_x86_64slc6gcc62opt64BS6G62AthenaOpt/NICOS_Log_2019-01-26T0957/PhysicsAnalysis_DerivationFramework_DerivationFrameworkInDet.html

This can be checked by

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

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