New/improved support for initialization and finalization routines in ThreadPool

[jrmadsen]

• New ThreadPool constructor arguments taking initialization and finalization functors
  • init/fini functors are executed by each thread in pool after creation and before destruction, respectively
• TBB executes initialization and finalization functors
• Miscellaneous clang-tidy fixes
• Restored support for PTL_DEVELOPER_INSTALL option
  • Setting to OFF disables install of headers and cmake configuration
  • Explicit options PTL_INSTALL_HEADERS and PTL_INSTALL_CONFIG also available