Reorganise test code for multi-threading

• Factor out the configuration and histograms into new classes
  • Each thread has its own store of histograms
  • Histograms are accumulated and written out at the end
• Implement standard library and TBB versions of test code
  • Ensure initialisation of EvtGen object only performed once per thread
  • Configuration of threading setup via second JSON input
  • Configuration of FSR generator also moved to same 'config' JSON file
• Update CMake config to find thread and TBB libs
• Add printout of total and average per-event generation time