Add VertexSelector as an example algorithm

This adds a MicroAODAlgos directory and a vertex selector: abstract base class, example class, and factory.  These are used in DiPhotonProducer.cc and incorporated into the simple_Producer_test.py example.  The machinery is based on recently-improved code (for PF linkers, specifically) and is designed so that, once it's in place, writing a new algorithm plugin is trivial.  For example, writing a "LegacyVertexSelector" would require only adding a flashgg/MicroAODAlgos/plugins/LegacyVertexSelector.cc, compiling, and changing the VertexSelectorName in flashgg/MicroAODProducers/test/simple_Producer_test.py