Implement issue #361

[MarkusFrankATcernch]

BEGINRELEASENOTES

• resolves Create check if geometry and field are not empty #361
  The Detector object has a state Detector::state() with three values:

    /// The detector description states
    enum State   {
      /// The detector description object is freshly created. No geometry nothing.
      NOT_READY = 1<<0,
      /// The geometry is being created and loaded. (parsing ongoing)
      LOADING   = 1<<1,
      /// The geometry is loaded.
      READY     = 1<<2
    };

  It starts with NOT_READY, moves to LOADING once the geometry is opened and goes to READY once the geometry is closed. As suggested in the developers meeting: the initial field object is invalid and gets created only once the geometry is opened. As a corollary, the field may not be accessed before. Geometry parsers must take this behavior into account!

• Address some compiler warnings.

  • Mainly add override/final statements in header files.

• Implement a module to invoke python as a DD4hep plugin:
  invoked e.g. by: geoPluginRun -plugin DD4hep_Python -dd4hep -prompt

  geoPluginRun -plugin DD4hep_Python -help
  Usage: -plugin <name> -arg [-arg]                                                  
     name:   factory name     DD4hep_Python                                        
     -import  <string>        import a python module, making its classes available.
     -macro   <string>        load a python script as if it were a macro.          
     -exec    <string>        execute a python statement (e.g. import ROOT.    
     -eval    <string>        evaluate a python expression (e.g. 1+1)          
     -prompt                  enter an interactive python session (exit with ^D)   
     -dd4hep                  Equivalent to -exec "import dd4hep"                
     -help                    Show this online help message.                       
  
     Note: entries can be given multiple times and are executed in exactly the     
           order specified at the command line!                      
  

  Implementation wise the plugin is a simple CLI wrapper for TPython.

ENDRELEASENOTES