AsgTools: add SlotSpecificObj mock class and use in TrigDecisionTool
Add SG::SlotSpecificObj
dummy implementation to the AsgTools package to transparently being
able to use code in Athena and Analysis releases. This class has the same functionality as the one from AthenaKernel except that it manages one single "slot".
This greatly reduces the amount of #ifdef
s required in code using this class (see changes to TrigDecisionTool in second commit).