TrigDecisionTool: code optimization and cleanup

Some code cleanup (no functional changes) in TrigDecisionTool:

• CacheGlobalMemory: store LVL1Item and HLT::Chain by value instead of pointers
• ChainGroup: make constructor/destructor public (required to be able to store them by value)
• DecisionUnpacker*: adapt to changes in CacheGlobalMemory
• ExperimentalAndExpertMethods returns now a const pointer

Other changes:

• use new style properties and header initialization
• fix a few thread-checker warnings (statics, const, etc.)
• use smart pointers where possible
• modernize code (e.g. range-based loops)
• fix some cppcheck warnings

cc @tamartin