PPI: Add boolean test for debug modules

PPI: Fix PassivePackeSink: provide front() instead of back() members
PPI: Provide specializations for void-argument callbacks in Events and Route
PPI: Add notifyThrottle/notifyUnthrottle to EventDescriptor
PPI: Re-implement routing classes using a traits-based implementation
PPI: Add Route unit testing (mainly testing throttling)

PPI: Implement throttling
PPI: Implement ModuleManager and ppi::init
PPI: Implement ThresholdQueuing
PPI: Hugely extended routing classes

PPI: Some small bugfixes to make ppitest work (not only compile :-) )
PPI: Add DebugModules