Reorganization of Counters headers

• renamed GaudiKernel/Counters.h to Gaudi/Accumulators.h, as by the convention that file names should match the namespaces
• moved StatEntity from Gaudi/Accumulators.h to GaudiKernel/StatEntity.h
• fixed Gaudi/Chrono/Counters.h and Gaudi/Accumulators.h inclusion order issue (using forward declarations)
• move counters unit tests to a proper unit test

Closes #99 (closed)