SConscript.py

- installation support (to SCONS_INSTALL_PREFIX, default $HOME/.local)
- RPATH handling, similar to CMake, with option to install with RPATH
- macOS support (linkflags, RPATH)
- library versioning
- support for decoder standalone test/example programs
- support building against installation location
- SCons support for SDK and plugins
    User plugins now use a standard SConstruct that should not need
    to be modified, and a minimal SConscript.py
- Improved EVIO local build
   * removed platform dependences
   * cleaned up lots of bad Python
   * build as part of decoder build, similar to CMake