Adding optimisations to the global trigger.

Adding optimisations to the global trigger.
- Trigger symbols with empty assignment expressions derive their values from the default expression and are effectively contant. They do not need to have their values
computed for every event, so they are now computed in the AliHLTGlobalTrigger::FillFromMenu method rather than the AliHLTGlobalTrigger::NewEvent method.
- Moving masking of CTP readout list to AliHLTTrigger::TriggerEvent so that the relatively expensive AliHLTTriggerDomain::operator AliHLTReadoutList () call is only
performed once.
- Avoiding expensive TFormula evaluations in AliHLTGlobalTriggerComponent::AddCTPDecisions.
- The global trigger logic will now properly handle all sub-classes of AliHLTTriggerDecision.
- Some minor improvements to AliHLTTriggerDomain and AliHLTDomainEntry.